If the historical evidence of the Catholic Church for the stability of the faith in the See and the Successor of Peter be not sufficient to prove, as a fact of history, that the Christian Church has so held and taught, history is altogether a poor and slender foundation for the events and actions of the past, The pretentious historical criticism of these days has prevailed, and will prevail, to undermine the peace and the confidence, and even the faith of some. But the City seated on a hill is still there, high and out of reach. It cannot be hid, and is its own evidence, anterior to its history and independent of it. Its history is to be learned of itself.
-Cardinal Edward Manning,:
The oecumenical council and the infallibility of the Roman pontiff : a pastoral letter to the clergy, &c (1869)
This post will be an exposition of the theological development of the concept of papal infallibility. Its main thesis is that papal infallibility is an inference or derivation from the concept of papal supremacy. Once this main thesis is realised the problematic nature of the dogma of papal infallibility would become a lot clearer.
The Key of Power versus the Key of Knowledge: Medieval Approaches to Papal Power
While it is common pop Roman Catholic apologetics to assert that the political authority of clerics, especially the popes, mediates our knowledge and understanding of the Scriptures or divine revelation, that was not the medieval consensus. The medieval world accepted the unmediated supremacy of the Scriptures with regards our knowledge of the contents of divine revelation and the articles of faith to be believed. But before we can begin our discussion on this point we need to understand the medieval distinction between the Key of Knowledge and the Key of Power.
The Key of Knowledge, or Lack Thereof
Pope John XXII in his bull
Quia Quorundam engages the question as to whether the Pope can define matters infallibly by the "key of knowledge" and categorically rejects it. In order for him to make his argument he has to distinguish between the the "key of power", which the popes and priests have, and the "key of knowledge", which they do not.
Addressing the "assertors", or his objectors, John says:
First, indeed, according to those who hold that the spiritual key is by no means knowledge, but the power to bind and loose, it is clear that the before mentioned assertors, in stating that it is knowledge, have erred. The definition the learned give of the key supports them [i.e. those who hold that the key is power]: "It is a special power of binding and loosing, by which the ecclesiastical judge should admit the worthy, and exclude the unworthy from the Kingdom”.
John XXII here is quite clear that the spiritual key of binding and loosing concerns only the power of excommunication and does not confer knowledge. He was even more explicit when he discusses the Matthew 16 passage dealing with Peter directly:
Our Saviour in the promise of the keys ["and I will give you the keys"] made to blessed Peter... added immediately after it: "And whatever you will bind upon earth will be bound also in heaven, and whatever you will loose upon earth will be loosed also in heaven"--making no mention of knowledge.
As far as John XXII understood it, with regards the power of binding and loosing, or the "keys" which Christ entrusted to Peter, it is merely the power of excommunicating and judging who is in or out of the Kingdom of God. That power to judge does not come with knowledge, infallible or otherwise.
The Bible as Supreme Epistemic Authority on Divine Revelation
Having distinguish the key of power from the key of knowledge, and shown that the power of binding and loosing concerned the power of excommunication and does not come with knowledge, it remains to show where is that knowledge of divine revelation to be found.
The consensus of the medieval Church was quite clear that the Bible was the supreme and unmediated epistemic authority over divine revelation. It alone provided supreme infallible knowledge of divine revelation. This was the repeated refrain from Aquinas which can be easily seen in the following quotes:
All the intermediaries through which faith comes to us are above suspicion. We believe the prophets and apostles because the Lord has been their witness by performing miracles, as Mark (16:20) says: “...and confirming the word with signs that followed.” And we believe the successors of the apostles and prophets only in so far as they tell us those things which the apostles and prophets have left in their writings.
- St Thomas Aquinas: De veritate Q14. Article 10. Reply 11
We have to observe the argumentative structure here carefully. He begins on the premise that
all the intermediaries of the faith are "above suspicion" or doubt, yet he says
only of the prophets and apostles that they are categorically to believed. The successors of the apostles on the other hand are only to be believed
only in so far as they communicate the prophets and apostles' writings. Thus, if the successors of the apostles are not above suspicion then
they are not the intermediaries through which faith comes. This is probably one of the clearest argument from Aquinas rejecting the idea that Roman clerics are intermediaries of the faith. In Aquinas's
Lectures on the Gospel of John he was likewise as explicit: "...only canonical Scripture is a measure of faith. Others, however, so wrote of the truth that they should not be believed save insofar as they say true things." In
Summa 1.1.8, while Aquinas speaks of the "authority of philosophers" as "extrinsic and probable" and the authority of the doctors of the Church as "merely probable", he speaks of the "authority of the canonical Scriptures as an incontrovertible proof". On the epistemic plane as such, the authority of the Scriptures is clearly the supreme and infallible basis for theological propositions.
To come back to John XXII, in the same bull quoted above he also makes a similar point in his arguments against his detractors. Challenging his detractors to prove their assertions, he argues:
Further, let them tell us where they read such assertions... Indeed, this pertains to faith neither directly, since there is no article about this nor any under which it can be comprehended–as is clear in the creeds, in which the articles of faith are contained–nor, also, [does it pertain to faith] reductively, as if scripture contains something like this, so that if it be denied the whole of sacred scripture is made doubtful and as a consequence the articles of faith, which have to be proved by sacred scripture, are made doubtful and uncertain.
(Bold and italics mine)
Thus John assumes as a premise that the articles of faith "have to be proved by sacred scripture". As such as far as knowledge goes; the proof or evidence for the truth of a theological proposition has as its basis the canonical Scriptures.
The Nature of the Key of Power
While the medieval Church was clear about the unmediated supremacy of the Scriptures over divine revelation, there arose the idea of the
political supremacy of the papacy which Aquinas explicitly taught as we can see here in his work
CONTRA ERRORES GRAECORUM:
It is also shown that the Vicar of Christ has universal jurisdiction over the entire Church of Christ...
It is also established... that the Roman Pontiff possesses a fullness of power in the Church.
(From Chapter 33 to 34)
While it has been acknowledged today that some of the quotes which Aquinas relies on are spurious, we need not trouble ourselves as to the historical basis for the claims of Aquinas. We need only content ourselves with the theological premises and principles which Aquinas held to, grounded or not, and the meaning and consequences of it.
We need to understand what Aquinas means here correctly. He is
not saying, for example, that only the Pope alone
knows what the Scriptures really mean, or that the Pope mediates divine revelation and alone has access to it. The Pope's power is
political and extends, supremely and absolutely, over the whole Church in the sense that he gets to rule, direct, and govern the Church by command or legislating or judging guilt or innocence. The political power to do such a thing however is not the same as saying that he has the knowledge to do so, even less infallible knowledge. In practical terms, all supreme political or legal power to do something simply means that his judgements are without appeal because there is no one else to appeal that judgement, and the orders of that power cannot be contradicted by any other higher authority or power.
As an example, a supreme court judge may have supreme legal power to judge cases and issue court order, etc. But even if he possesses such a supreme legal power, and as such his judgement is final, absolute, and there is no possibility of appeal against it, it doesn't mean that he is in possession of great legal knowledge nor does it mean that he cannot err in his judgement. The judge may still be a complete idiot, he may have judged the facts wrongly based on a faulty understanding of the law or situation, but he would still have such a supreme legal power if he is so appointed by the proper authorities. (A glance at the judgements of the United States Supreme Court, for example, would make clear the divergence between possessing supreme legal authority to judge things and the possession of actual wisdom/knowledge to do so.)
This point, with regards the pope, can be easily seen when we see how Aquinas explains what constitutes the Pope's power:
It is also shown that Peter is the Vicar of Christ and the Roman Pontiff is Peter’s successor enjoying the same power conferred on Peter by Christ. For the canon of the Council of Chalcedon says: “If any bishop is sentenced as guilty of infamy, he is free to appeal the sentence to the blessed bishop of old Rome, whom we have as Peter the rock of refuge, and to him alone, in the place of God, with unlimited power, is granted the authority to hear the appeal of a bishop accused of infamy in virtue of the keys given him by the Lord.” And further on: “And whatever has been decreed by him is to be held as from the vicar of the apostolic throne.”
It is quite clear here that by "unlimited power" it refers to the Pope's authority to be the final court of appeal for all disputes, in the same sense that United States Supreme Court is the final court of appeal for all cases. The Pope claims the right to hear all cases under his jurisdiction, which is the whole Church, and whose judgement on such cases is final and simply without appeal. There is no political authority higher than the pope to whom one can appeal.
But we have to note the sort of cases the Pope here is hearing. He is hearing a case about the "infamy" or guilt or innocence of a bishop with regards being sentenced or accused of something, the Pope is
not deciding a doctrinal truth; he is deciding a
legal point of whether a bishop is guilty of what he is accused of and liable to be sentenced. To be sure whether or not a bishop is guilty may involve doctrinal truth
if the bishop is being accused of heresy, then the pope, in the course of condemning or exonerating him, has to decide whether what he says is heresy, but that is not the main object of the pope's power. The pope's jurisdiction is precisely over a legal and political point of whether someone has been sentenced rightly or condemned justly, and only
incidentally would it involve doctrinal truth only if the bishop is accused of heresy. As such, the papal unlimited governing powers concerned discipline and political prudence, not as an epistemic source of knowledge concerning the faith. But I will expand on this point later.
Aquinas's most explicit expression of the distinction between the epistemic sufficiency of the Scriptures and the political nature of papal authority can be seen in Summa 2.2.1.10. There Aquinas discusses the question as to whether the Pope has the right to draft creeds.
"Objection 1. It would seem that it does not belong to the Sovereign Pontiff to draw up a symbol of faith. For a new edition of the symbol becomes necessary in order to explain the articles of faith, as stated above (Article 9). Now, in the Old Testament, the articles of faith were more and more explained as time went on, by reason of the truth of faith becoming clearer through greater nearness to Christ, as stated above (Article 7). Since then this reason ceased with the advent of the New Law, there is no need for the articles of faith to be more and more explicit. Therefore it does not seem to belong to the authority of the Sovereign Pontiff to draw up a new edition of the symbol.
[...]
Reply to Objection 1. The truth of faith is sufficiently explicit in the teaching of Christ and the apostles. But since, according to 2 Peter 3:16, some men are so evil-minded as to pervert the apostolic teaching and other doctrines and Scriptures to their own destruction, it was necessary as time went on to express the faith more explicitly against the errors which arose."
One needs to observe the arguments here. Aquinas does not appeal, contra contemporary high church apologists, to the skeptical idea that the Scriptures are ambiguous, unclear, that we can't discern its meaning without some Magisterium. Rather, there is an outright positive affirmation that the teachings of Christ and the Apostles are sufficiently clear, it is epistemically sufficient. The point of creeds however is
not to enlighten the ignorant but to
discipline the the evil who deliberately pervert the truth. As such, the point of creeds is political, prudential and disciplinary, to discipline and rebuke the wilful heretic, not to uncover truths which we have no access to without the Magisterium.
The tension however comes in because Aquinas seems to think that the Pope also has the political authority to decide controversies about the faith. In his own words: "... to the aforesaid Pontiff belongs the right of deciding what pertains to faith." The problematic question is: Can the Pope decide matters of faith without at the same time possessing infallible knowledge to do so?
The Extent of Papal Infallibility in the Use of his Supreme Power
We have seen that the medieval Church, at the same time, held to the epistemic supremacy of the Scriptures, that it contains infallible and absolute theological truth, while holding unto the political supremacy of the pope. The question which naturally arises out of this is: Does the Pope need to have infallible knowledge in other to exercise his supreme political power? To better appreciate this question we will look at three cases: cases of excommunication, canonisations, and finally the issuing of creeds.
Sentences of Excommunication
If we remember from our previous quote from John XXII, he explicitly identified the power of excommunication as what specifically pertains to the Key of Power, the Power to admit and reject from the Kingdom of God. The Pope as such presumably is supreme politically, "the last court of appeal" and where the buck stops when it comes to sentences of excommunication.
As we have discussed, when the Pope declares, say, a cleric excommunicated or defrocked on the basis of sexual misconduct, his judgement is final and without appeal. There is no other power capable of contradicting him and there is no other court above him (except maybe the Last Judgement itself). The problem is, is the Pope infallible when he passes such sentences of excommunication? The answer has to be, definitely not, even on present Roman Catholic premises.
An argument against the infallibility of the papal excommunication sentences is as follows: Whether a specific person is guilty of a specific crime is an empirical question, an empirical fact which doesn't properly belong to the apostolic Deposit of Faith passed down. The Apostles may have taught that sexual abuse is a sin, and the Pope maybe able to infallibly pass down what the Apostles taught (we will come to this point later). But the Apostles cannot possibly teach that "Father Gomez is guilty of sexually abusing altar boys" for no other reason than that the apostles were not even aware of the existence of Father Gomez who lived two thousand years after they died! As such the Pope cannot be infallible when it comes to sentences of excommunication even if he has supreme political power over the entire church and no one may contradict him. The Pope maybe the successor of St Peter, but unlike his predecessor of old he is not gifted with supernatural insight to know whether or not Ananias and Sapphira were lying to him (we will also come back to this point). As such, the Pope is not privy to such empirical facts. In Roman theology today papal infallibility does not extent to his "ordinary" exercise of power, supreme though it maybe, which commands cannot be overruled, whose judgement may not be appealed.
Cases of Canonisations
While this case is pretty straight forward, the difficulty sets in when it comes to more "important" matters concerning his governance, such as the canonisation of saints. If the Pope is not specially privy to the facts concerning whether Father Gomez is a sexual abuser, is the Pope specially privy to the facts as to whether a particular person is orthodox in his faith, has died in the state of grace with no secret sin, and has truly performed such and such number of miracles? The difficulty can be seen at once. If the Pope could make a mistake in canonising saints then an entire system of piety, the faith and trust of its faithful, will be thrown into a crisis of doubt. Imagine a lifetime invoking the help of someone and he turns out to be roasting in hell or purgatory. However Catholic theology is clear that as far as the infallibility of the Church is concerned it can only extend to matters of apostolic tradition and the deposit of faith. They can only communicate, without fail, what the apostles taught and divinely revealed. There is however no promises or guarantee regarding the Church's infallibility concerning empirical matters of facts.
Aquinas felt the problem, and although he has repeatedly argued that the Scriptures were supreme epistemically and that all other authorities are to be believed only in so far as they say what is true, he saw that the piety of the Church would be threatened if there was no certainty concerning canonisation of saints. And thus he asserts, in the medieval fashion, that piety requires that the Pope be infallible when it comes to canonisation, although he had the cop out that he is so only in so far as he represents the Church. The exact quote from Quodlib. IX, a. 16 is: "Since the honor we pay the saints is in a certain sense a profession of faith, i.e., a belief in the glory of the Saints, we must piously believe that in this matter also the Church is not liable to error." The appeal here is to "piety", not Scripture or even Tradition. Such a reasoning is a species of the "conclusion theology" or "fitting argumentation" that if something was "fitting" God would do it. It is unfitting that God would allow millions to invoke people roasting in hell, ergo God must have guaranteed the judgements of popes concerning sainthood.
While we can understand the motivation behind the argument, the problem has not been solved at all. The medieval Church may need papal infallibility in canonisation to justify their piety in the saints, but just because they need it for their piety is God obliged to supply it? Maybe the proper response to such uncertain piety is to qualify our trust, not invent new promises unknown to Scripture and tradition out of the thin air. As such, it is impossible to square the Pope's miraculous infallibility when it comes to saintly canonisations with the repeated assertion that the Pope simply does not possess the key of knowledge and the repeated rejection that infallible knowledge comes with clerical charisma.
Issuing of Creeds
The tension however becomes critical when it comes to the promulgation of creeds. As part of his duty as the supreme governor of the Church the Pope has the power to issue and promulgate creeds. As part of that same governing power the Pope has of course the power to excommunicate people for heresy or rejecting the creeds which contains divinely revealed articles of faith.
However, while it is clear that sentences of excommunication concerning empirical misconduct of sexual abuse is not infallibly guaranteed, are excommunication over heresy likewise open to the same fallibility? The medieval tension is clearest here: The Scriptures alone are the supreme epistemic authority concering divine revelation. Aquinas was explicit on this point that the successors of the apostles were to be believed only in so far as they teach in accordance with the Scriptures. Pope John XXII in his condemnation of papal infallibility in Quia Quorundam, where he distinguishes the Key of Power from the Key of Knowledge, was also explicit that articles of faith must be demonstrated from the Scriptures. Thus, prima facie, we are forced to conclude that the Pope's command to receive a certain creed as containing true theological propositions has to be subjected to the Scriptures. Lacking the key of knowledge, which has been committed to the Church as a whole and contained supremely in the Bible, the Pope's political power cannot override the Scriptures concerning divine revelation.
Yet how could the Pope exercise his supreme authority to govern the Church if whether or not to believe the theological content of his creeds and his teachings are subjected to Scriptural checks? If upon examination of the Scriptures we see that the successor of Peter has issued a creed which contains theological errors, is the Church bound to accept his erroneous creed in the same way that the Church is supposedly bound to accept his sentences of excommunication, mistaken as they maybe?
The medieval tension in fact has never been solved; it has simply resulted in the Reformation with the Protestant Reformers simply accepting the epistemic supremacy of the Scriptures, as taught by the medieval Church, and asserting outright the fallibility of the political commands of clerics and our freedom to dissent from them. The Roman position continued to hold them together, with numerous problems and paradoxes along the way.
The Critical Clash between the Pope's Lack of Knowledge and His Supreme Power
The post-Reformation age did not solve the medieval problems. In fact it heightened is until it reached various critical points. To see where the critical clash occurred we need only look at a curious 17th century controversy within the Roman Church. For our present purposes we need only give a sketch of the controversy.
In the 17th century a Jesuit doctoral thesis advanced the following proposition: We can only have moral, not infallible, certainty as to whether or not a specific man is the pope. The argument is as follows: Whether or not a certain man is the Pope is dependent upon certain historical and empirically sensitive facts like whether or not he has been elected without simony or has been validly baptised. Whether or not a specific man, who lived hundreds of years after the apostles's death, is validly baptised cannot possibly be part of the original deposit of faith. As such, although we can be morally certain, it cannot be infallibly guaranteed part of the original deposit of divine revelation.
The uproar which this thesis provoked was followed by an even more rigorous attempt to refute the thesis. On such a critical matter such as whether or not a specific man is the pope they felt an "existential" need for infallible certainty.
Yet all the Catholic Church accepted that divine revelation ceased with the apostles. Whether or not divine revelation can be found only in the Scriptures or in both the Scriptures and tradition, it cannot possibly be asserted on either option that what happens hundreds of years after the apostles died is part of the original apostolic teaching.
A bold solution to this problem was to argue as follows: It is part of the original divine revelation that papal definitions are divinely revealed. Ergo, if the pope defines that such and such person is the pope, that fact becomes "equivalent" to divine revelation. Most Roman theologians pointed out that this obviously broke the principle that there cannot possibly be new divine revelation. However what is of interest to us is this: Can political exercises of papal power in defining matters become "divine revelations" simply by virtue of the exercise of his power? As it turns out, Vatican I's answer is essentially: Yes.
Vatican I Non-Epistemic Arguments for Papal Infallibility
Vatican I can be considered to be a "resolution" to the medieval problem, but it resolves it in effect by ignoring it. My contention would be that it doesn't address any of the points of the epistemic deficiency of the Pope, it addressed it by taking the need for efficacious papal power to its logical conclusion: If it is to be effective, it has to be infallible. Papal infallibility, as such, is not based on any epistemic arguments, but is derivative of the need for effective exercise of papal power.
Despite pop Roman Catholic attempts to argue that the papacy can give us epistemic certainty concerning the contents of divine revelation, that is most certainly not the premise or argument of Vatican I.
Vatican I continues to operate on the "realist" premises of the medieval Church, that is, divine revelation, both natural and supernatural, is intelligible, and can be known with certainty to our reason. Here are some passages from Vatican I:
The same Holy mother Church holds and teaches that God, the source and end of all things, can be known with certainty from the consideration of created things, by the natural power of human reason...
Nevertheless, in order that the submission of our faith should be in accordance with reason, it was God's will that there should be linked to the internal assistance of the Holy Spirit external indications of his revelation, that is to say divine acts, and first and foremost miracles and prophecies, which clearly demonstrating as they do the omnipotence and infinite knowledge of God, are the most certain signs of revelation and are suited to the understanding of all.
5. Hence Moses and the prophets, and especially Christ our lord himself, worked many absolutely clear miracles and delivered prophecies; while of the apostles we read: And they went forth and preached every, while the Lord worked with them and confirmed the message by the signs that attended it. Again it is written: We have the prophetic word made more sure; you will do well to pay attention to this as to a lamp shining in a dark place.
Note well that not only God can be known by the "natural power of human reason", but that divine revelation can be known "clearly" with "certainty" via miracles, signs and wonders, prophecies, etc. Vatican I doesn't say, as per pop Roman apologetics, that divine revelation is intrinsically opaque, that we cannot know it without the Magisterium, or that it is knowledge "mediated" by the Pope. In fact Vatican I goes even further that these things can be known with
certainty and absolute clarity by the divine acts of miracles and prophecies.
If Vatican I, and all of the Church before, were not obsessed with epistemic questions about the inherent ambiguity or unknowability of divine revelation, why then papal infallibility?
If one looks at the structure of the argument of Vatican I, the reason is obvious: Papal infallibility is the consequence for
effective political exercise of papal power. While most Roman Catholic and Protestant disputes are centered on papal infallibility, the logical order in fact is that of first papal supremacy then papal infallibility which is derived from the former. This can be most clearly seen in the argumentative structure of the
First dogmatic constitution on the Church of Christ of Vatican I.
Chapter 1 sets out the foundational principle from which papal infallibility would be derived:
Chapter 1
On the institution of the apostolic primacy in blessed Peter
1. We teach and declare that, according to the gospel evidence, a primacy of jurisdiction over the whole Church of God was immediately and directly promised to the blessed apostle Peter and conferred on him by Christ the lord.
[...]
3. And it was to Peter alone that Jesus, after his resurrection, confided the jurisdiction of Supreme Pastor and ruler of his whole fold, saying:
Feed my lambs, feed my sheep.
The premise is obvious: Christ conferred upon Peter, and presumably his successors, this supreme "jurisdiction" to rule over the whole Church. The political power of the Pope is the foundation for all the subsequent arguments. Papal infallibility is explained only in the last chapter and in these terms:
Chapter 4.
On the infallible teaching authority of the Roman Pontiff
1. That apostolic primacy which the Roman Pontiff possesses as successor of Peter, the prince of the apostles, includes also the supreme power of teaching. This Holy See has always maintained this, the constant custom of the Church demonstrates it, and the ecumenical councils, particularly those in which East and West met in the union of faith and charity, have declared it.
[...]
Then there is the definition of the Council of Florence:
"The Roman Pontiff is the true vicar of Christ, the head of the whole Church and the father and teacher of all Christians; and to him was committed in blessed Peter, by our lord Jesus Christ, the full power of tending, ruling and governing the whole Church." [58]
3. To satisfy this pastoral office, our predecessors strove unwearyingly that the saving teaching of Christ should be spread among all the peoples of the world; and with equal care they made sure that it should be kept pure and uncontaminated wherever it was received.
[...]
7. This gift of truth and never-failing faith was therefore divinely conferred on Peter and his successors in this See so that they might discharge their exalted office for the salvation of all, and so that the whole flock of Christ might be kept away by them from the poisonous food of error and be nourished with the sustenance of heavenly doctrine. Thus the tendency to schism is removed and the whole Church is preserved in unity, and, resting on its foundation, can stand firm against the gates of hell.
8. But since in this very age when the salutary effectiveness of the apostolic office is most especially needed, not a few are to be found who disparage its authority, we judge it absolutely necessary to affirm solemnly the prerogative which the only-begotten Son of God was pleased to attach to the supreme pastoral office.
After this follows the definition of papal infallibility.
The argument as such can be summarised in the following deduction:
(1) Christ entrusted Peter and his successors with the Key of Power to govern the Church with full, supreme, final and absolute authority.
(2) In order to do (1) effectively the Pope has to be infallible
on some occasions of the exercise of his authority, namely, when he is teaching, etc. The argument is clearest in 7. where it is explicitly pointed out that the "gift of truth and never-failing faith" was "divinely conferred" on the pope "
so that they might discharge their exalted office for the salvation of all, etc."
As such, the reason for papal infallibility isn't due to any epistemic arguments pop apologists are so fond of that the pope has to mediate divine revelation otherwise it is unintelligible, unknowable, or even that it cannot be known with certainty; the reason for papal infallibility is to that the pope can be a more effective political ruler, a political mission or office which Christ promise to St Peter and his successors. That is, to prevent schism, foster unity, and discipline the erring. It has nothing to do with epistemic issues of revealing impenetrable or inaccessible divine revelation. Vatican I's answer to the medieval problem is in effect: We will overcome our lack of knowledge with power.
Despite the deductive elegance of this argument, there is one basic problem: It is pure speculation, a mere product of "conclusion theology". It is solving a problem purely of its own invention. We Protestants have a simple modus tollens solution to the medieval problem of whether the pope can discharge his supreme governing responsibilities without infallible knowledge: Since the Pope does not possess the key of knowledge and since he can be mistaken concerning heresies ergo he cannot possibly possess supreme political power to govern the Church because he can issue commands to believe in theological mistakes. With this we conclude with some objections to the "development" of papal infallibility.
Conclusion: No Dividing the Key of Power
Papal infallibility clearly is a conclusion twice removed from the direct biblical datum. On Vatican I premises, Christ supposedly gave St Peter the supreme power to govern the whole Church, papal infallibility is inferred from that granted power on the premise that he needs papal infallibility to do it effectively.
For the sake of argument, even if we grant that all the biblical and patristics writings concedes to the Pope supreme political power to govern the whole Church, and we most certainly do not, it still doesn't follow that the Pope has to be infallible in every exercise of that power. He can after all still make mistakes about excommunication sentences. Nothing in the Fathers or the Bible says that the Pope has to be infallible every time he exercises his power. Not even Romanist today believe this.
As we have seen, the problem already emerged in the medieval Church when Aquinas simply asserted that the Pope was infallible in canonisation because the Church needed certainty for their piety. That argument however is mere speculation, it invents a divine promise just to meet a postulated need.
The post-Reformation Roman Church likewise struggled with trying to determine when and how the Pope was infallible. Many various theories were postulated but all of them were merely speculative. In the end Vatican I is merely the product of "conclusion theology". Without justification or grounds from Scripture or Tradition, they simply invented a promise which pertains to specific criteria and scope for infallibility based on some systematic consideration and felt need for how papal supremacy can work coherently in the real world.
What is even more confusing is that contemporary Roman theologians who hold that canonisations are infallible are explicit in distinguishing papal infallibility regarding canonisations from papal infallibility as defined in Vatican I. They claim that papal infallibility is derived directly from the pope's efficacious and supreme governing power. But this is completely arbitrary. If the pope needs to be infallible in canonisations in order to effectively govern the church, why shouldn't he need to be infallible in excommunications in his governance of the church?
Our Protestant objection therefore is that the selection and demarcation of only specific exercises of papal power to be infallible is arbitrary from the standpoint of the Bible.
If truly the Pope were entrusted with such a Key of Power with such and such promises pertaining to the Key there is simply no biblical or even traditional reason to believe that the promises pertain with varying strength to the use of the key. Either all exercises of it are infallible or none are. But it is not like there is a switch on the key to use on "half power" when deciding "ordinary matters" and then the Pope can ramp it up to "full power" when infallibly deciding matters of doctrine of faith and morals. The distinction is pulled completely out of the thin air.
If we look at the biblical witness, St Peter in deciding how to distribute the donations knew supernaturally that Ananias and Sapphira were lying to him. This infallible insight doesn't pertain to a doctrinal definition of faith and morals, it is literally knowledge concerning a purely prudential matter. Why should not his subsequent successors be likewise gifted with such infallibility when even deciding ordinary matters? The Pope as such should be, for example, infallible in his excommunication sentences, he should supernaturally be able to know which priests under him are engaging in kiddy fiddling and be infallible in defrocking the whole lot. When deciding what books to put on the Index he should be able to know that Kepler's book contains accurate astronomical data.
Of course the unfortunate fact is that the subsequent history shows that this cannot be sustained. But if the premise of papal supremacy doesn't entail in a straightforward logical manner the Pope's infallibility in every exercise of his supreme power given by Christ, on what basis do they invent such a distinction to say that he is infallible only on this occasion and not that?
The conclusion which we Protestant contend is that there is no reason whatsoever. This distinction is a pure concoction and summoned completely out of the thin air. Its purpose is not to unfold the meaning of Scripture or even tradition. Its main purpose is presentist propping up of the Vatican institution. As such logically to be consistent if the promise of Christ pertains to the keys as a whole he should be infallible all the time. Otherwise, they don't get to put a power level switch on it just because history can't bear out their logic.
While most apologetics today is focused on papal infallibility, the historical facts shows that papal infallibility is merely an inference from papal supremacy. However that the major premise leads to the minor is by no means self-evident. It is merely the product of an old but ultimately dangerous medieval principle: If something is "fitting" God would do it. No principle has generated more heresies than this. Probably half of the errors of Rome can be traced back to this idea; if I can formulate a cool story as to why it would be nice if something is true, it totally has to be. The proper response to this should be: Cool story bro.
