There is a passage from the Summa which is frequently appealed to by Roman apologists which goes:

Now the formal object of faith is the First Truth, as manifested in Holy Writ and the teaching of the Church, which proceeds from the First Truth. Consequently whoever does not adhere, as to an infallible and Divine rule, to the teaching of the Church, which proceeds from the First Truth manifested in Holy Writ, has not the habit of faith, but holds that which is of faith otherwise than by faith.

– S.T. 2.2.5.3

Thus the moment they see “adhere, as to an infallible and Divine rule, to the teaching of the Church”, they instantly infer infallibility to obtain of the Magisterium of the Church.

But there is no justification for that inference for two reasons (1) Aquinas is referring to the propositional content, the things or realities referred to by the teachings, the teaching of the Church is not to be confused with the teachers of the Church; it is the teachings of the Church which is infallible and divine, not the teachers of the Church, (2) Aquinas does not see, in this very particular context, infallibility as something which obtains of the agent who promulgates the First Truth, but rather infallibility is something which obtains of the logical deduction from the First Truth, that is to say, the teachings of the Church which logically and ontologically “proceeds from the First Truth”, not what is simply proclaimed by Magisterial Authority.

To motivate the proof let’s consider the following analogy. The differentiation rule is a teaching of my mathematics teacher, it is an infallible rule for deriving differentiations. However even though the teaching itself, that is, the differentiation rule, is an infallible rule for deriving differentiations in that the rule applied correctly will not fail to derive proper differentiations, it does not follow that the teacher of the rule, my mathematics teacher, is infallible or can never err in his teachings. It is the propositional content itself which is the infallible rule, not the teacher.

To come back to Aquinas, it might be easier to explain (2) first, to illustrate the metaphysical and ontological principles which determines the meaning of “infallible”, before moving on to (1).

Let’s look at what Aquinas says about the knowability of first principles and its logical relation to the First Truth, which, as we shall see later, Aquinas identifies with God. In Quodlibet X. q4. a. I he states:

We cannot know anything about the truth except on the basis of the first principles and in the light of the intellect, which cannot manifest the truth except inasmuch as they are a likeness to the first truth, since from it they have a certain immutability and infallibility. All truths are not seen in this life in the first truth according to essence, given that neither is the first truth seen by us in this life in its essence. However, every truth is known in it on account of its image, that is, on account of the truth copied from it.

(italics, underline, bold mine)

This is a remarkable passage because it shows the metaphysical and realist background Aquinas is operating on which determines the “infallibility” of first principles. It is “infallible” not because it has been promulgated by the authority of the Magisterium of the Church or the promise of Christ to the successors of Peter; its infallibility is derivative of its ontological and metaphysical resemblance to God, the First Truth. By “resembling” the First Truth, first principles acquire bits of its immutability and infallibility. As such, infallibility is a relationship which obtains between the different realities which logically, or more accurately, ontologically flows from the First Truth, that is to say, it is a relationship which obtains between the propositional content of the “teachings of the Church”. It has nothing to do with the Magisterium being able to provide infallible epistemic certainty.

We are now in a position to explain (1). The quote from Quodlibet parallels almost exactly that of the Summa quote at the beginning. Just as first principles and the light of the intellect cannot “manifest” the truth unless it has a certain likeness to the First Truth from which it acquire its “infallibility”, likewise the “formal object of faith” which is “manifested” in the Holy Scriptures and the teaching of the Church acquires its “infallibility” by likeness or some ontological relationship to the First Truth, not by promulgation from an infallible Magisterium. As such, if the propositional content, the realities referred to by the teaching of the Church, is what is “infallible”, the natural question which we have to ask is, which specific teaching or realities does Aquinas refer to which he wants us to treat as an infallible and divine rule? The answer is not far to find.

Aquinas is asking a very particular question in 2.2. art. 5: “Whether a man who disbelieves one article of faith, can have lifeless faith in the other articles?” Aquinas response with: “Neither living nor lifeless faith remains in a heretic who disbelieves one article of faith…” The point as such is that a person who disbelieved one particular article of faith does not possess the habit of faith, just as whoever does not follow the differentiation rule would not be able to derive differentiations. As such this isn’t about the authority of the Magisterium but about the specific content of belief or unbelief. What specific article of faith which if one denies will deprive one of the virtue of faith? To answer this we have to know what Aquinas means by an “article of faith.”

The answer can be found in Question 1 of the very same section. Here Aquinas identifies the “First Truth”–the “formal object of faith” referred to in q. 5–with Deity itself (art. 1). However, the things to which faith assents includes not only God, but things ontologically and logically related to God, specifically those divine operations that aid the human being on the way to salvation. He specifies in particular: “Things concerning Christ’s human nature, and the sacraments of the Church, or any creatures whatever, come under faith, in so far as by them we are directed to God, and in as much as we assent to them on account of the Divine Truth.”

This is strongly confirmed in his argument in q. 1, art. 8 in response to objection 5 that the “articles of faith” are unsatisfactory because the Eucharist is not mentioned. Aquinas summarizes the articles as follows:

Now with regard to the majesty of the Godhead, three things are proposed to our belief: first, the unity of the Godhead, to which the first article refers; secondly, the trinity of the Persons, to which three articles refer, corresponding to the three Persons; and thirdly, the works proper to the Godhead, the first of which refers to the order of nature, in relation to which the article about the creation is proposed to us; the second refers to the order of grace, in relation to which all matters concerning the sanctification of man are included in one article; while the third refers to the order of glory, and in relation to this another article is proposed to us concerning the resurrection of the dead and life everlasting. Thus there are seven articles referring to the Godhead.

In like manner, with regard to Christ’s human nature, there are seven articles, the first of which refers to Christ’s incarnation or conception; the second, to His virginal birth; the third, to His Passion, death and burial; the fourth, to His descent into hell; the fifth, to His resurrection; the sixth, to His ascension; the seventh, to His coming for the judgment, so that in all there are fourteen articles.

The “articles of faith” here is decidedly not “whatever the Magisterium says”, the articles of faith are simply identified with the subject matter, or propositional content of the Apostles’ and Nicene Creeds, the realities to which the creed refers to. This once more confirmed if we go back to 2.2.5.3 where Aquinas later says: “…if, of the things taught by the Church, he holds what he chooses to hold, and rejects what he chooses to reject, he no longer adheres to the teaching of the Church as to an infallible rule, but to his own will.” He once more here refers again to the subject matter, the things taught by the Church, as what is properly the infallible rule.

As such we conclude this examination of Aquinas on Infallibility by contending that most Roman apologetics project backwards unto Aquinas early modern and Cartesian obsessions about infallible certainty and epistemic security. Aquinas on the other hand, when using terms like “infallible” refers instead to a metaphysical resemblance or likeness between realities referred to by the teachings of the Church. Aquinas was simply not interested in early modern or contemporary obsessions about using the Magisterium to solve epistemological problems. As such, despite the initial rhetorical force of the quotes from Aquinas as seeming to support the Roman apologetic, a close reading of Aquinas’s arguments, and how he uses the terms in question, reveals that the apologetic is projecting very different premises and systems back into Aquinas.

P.S. The content for the argument for (1) was mostly provided by William Witt here. His post helped me realise the propositional nature of Aquinas’s argument. The finding of the Quodlibet quote was something of a bit of luck when I was researching for another topic, but it confirms strongly that Witt’s reading is essentially the correct one.