"The precepts of the Law, which man is bound to fulfil, concern acts of virtue which are the means of attaining salvation. Now an act of virtue, as stated above (Q, A) depends on the relation of the habit to its object. Again two things may be considered in the object of any virtue; namely, that which is the proper and direct object of that virtue, and that which is accidental and consequent to the object properly so called. Thus it belongs properly and directly to the object of fortitude, to face the dangers of death, and to charge at the foe with danger to oneself, for the sake of the common good: yet that, in a just war, a man be armed, or strike another with his sword, and so forth, is reduced to the object of fortitude, but indirectly.(Thomas Aquinas, Summa Theologica, Part 2 of Part 2 (sic), Q.1, A. 9).
Accordingly, just as a virtuous act is required for the fulfilment of a precept, so is it necessary that the virtuous act should terminate in its proper and direct object: but, on the other hand, the fulfilment of the precept does not require that a virtuous act should terminate in those things which have an accidental or secondary relation to the proper and direct object of that virtue, except in certain places and at certain times. We must, therefore, say that the direct object of faith is that whereby man is made one of the Blessed, as stated above (Q, A): while the indirect and secondary object comprises all things delivered by God to us in Holy Writ, for instance that Abraham had two sons, that David was the son of Jesse, and so forth.
Therefore, as regards the primary points or articles of faith, man is bound to believe them, just as he is bound to have faith; but as to other points of faith, man is not bound to believe them explicitly, but only implicitly, or to be ready to believe them, in so far as he is prepared to believe whatever is contained in the Divine Scriptures. Then alone is he bound to believe such things explicitly, when it is clear to him that they are contained in the doctrine of faith"
The language of Aquinas and his scholastic methodology may be foreign to Lutherans -we're certainly uncomfortable with discussing faith under the rubric of Law, of what we are "bound" to believe, preferring to regard faith as a gift - but Aquinas's conclusion here is something we could assent to. I've included the long first paragraph only because it is necessary to understand Aquinas's conclusion in this matter, which is that we are "bound" to believe in the Holy Trinity and the mystery of Christ (he has previously been discussing these points) as they are the "proper and direct objects" of faith by which we are blessed with salvation; and then also we are to be prepared to believe "whatever is contained in the Divine Scriptures", even down to historical details.
Now, that is a very interesting statement in itself, a matter which post-Enlightenment Christians have been contending about for two hundred years now, but I want to leave it for the present and move on to the final sentence: we are bound to believe such things "explicitly" - and I think Aquinas here means by virtue of their being revealed by God - when it is clear to (us) that they are contained in the doctrine of faith [italics mine]. What is the doctrine of faith? Elsewhere Aquinas states "the truth of faith is contained in Holy Scripture" (ST, 2, 2 Q1, A9), and "Holy Scripture is the rule of faith" (Commentary on the Sentences [of Peter Lombard], 3.25).
What can we conclude from this? Well, under pain of ecclesiastical obedience, I don't want to turn this towards the question of the perpetual virginity of Mary (which Aquinas believed in), except to say that, if we consistently follow Aquinas's principles, belief in such can only follow upon a person being convinced that it is taught in Holy Scripture. Nor do I want to contend that Aquinas is some sort of proto-Lutheran, that would be anachronistic. But he does, to my mind, most certainly teach the primacy of scripture over the church and its sufficiency as a rule of faith, which effectively means that the church can only propose for belief what is contained "explicitly or implicitly", in Holy Scripture.
In support of this conclusion, I note that when, again in his Commentary on the Sentences (of Peter Lombard) - a standard medieval theological text - Aquinas answers a supposed objection that "if Holy Scripture is a sufficient rule of faith to which no addition or from which subtraction is permitted then the drawing up of creeds is superfluous", he replies to the effect that in the Creeds the Fathers only summarised what was explicit in scripture, as occasioned by the doctrinal controversies they were facing:
"The Fathers who have published other symbols after the Apostles have not added anything of their own, but added what they excerpted from the Holy Scriptures. Now, since in that symbol of the Apostles there are some difficult things, the Nicene Creed was published, which exposes more fully the faith about certain items. Since then some truths were contained in those symbols in implicit form, it was necessary to give an explanation upon the rise of heresies, and so was added the symbol S. Athanasius, who especially set himself against the heretics."Note that Aquinas assumes the correctness of the basis of the objection, and only explains that creeds don't really add to the rule of faith, but only summarise it. If any further evidence for Aquinas's views on this were needed, then we have the following from the first part of the Summa (1.1.8), where Aquinas responds to the objection that articles of faith are merely arguments from authority, and therefore intrinsically weak [italics mine]...
"...sacred doctrine makes use of these authorities as extrinsic and probable arguments; but properly uses the authority of the canonical Scriptures as an incontrovertible proof, and the authority of the doctors of the Church as one that may properly be used, yet merely as probable. For our faith rests upon the revelation made to the apostles and prophets who wrote the canonical books, and not on the revelations (if any such there are) made to other doctors."
Note that Aquinas refutes the objector by establishing that articles of faith are based on divine revelation and authority, which has been delegated (my words) to the prophets and apostles who authored (in the wide sense) Holy Scripture. The church then makes use of human authorities, such as the Fathers, only as providing "probable" evidence for doctrine. The term "probable" here is used in a technical philosphical/epistemological sense pertaining to levels of certainty in our knowledge - the highest degree of certainty is said to be apodeictic - it can be conclusively demonstrated, in the case of doctrine this demonstration is provided from scripture; the lowest level of certainty is probability, which is only as strong as the evidence a "probable argument" musters. Aquinas, it would seem then, would be loathe to establish a doctrine on the basis of anything other than a clear word or logical inference from the divinely inspired and hence supremely authoritative Holy Scripture - contra the modern papacy, which has three times proclaimed as doctrine articles of faith which cannot be found in scripture (Immaculate Conception of BVM, 1854; Infallibility of Pope, 1870; Assumption of BVM, 1950).
Now, this assertion might bring forth an objection from a Roman Catholic to the effect that Aquinas defends the right of the Supreme Pontiff (i.e. the Pope) to call a Council and add to a symbol of the faith (i.e. what is to be believed by Christians). True enough, but note first that Aquinas states that the Pontiff has that right "in order to set aside errors that may arise"; therefore, I suggest, Aquinas conceives this of power as being defensive, related to refuting error, not proposing new dogmas on the basis of revelations found outside of scripture "if such there be". And, secondly, note the basis on which Aquinas grounds this power of the Supreme Pontiff - the Decretals. That is a resort to mere ecclesiastical positivism; simply because the Pope says he has the right to do something does not make it so. This is, in my opinion, the fatal error of Aquinas, by which, because of his incredible influence upon subsequent theology and ecclesiology, he opened the door to the corruption of the catholic faith by the enthusiasm** of the Papacy.
*Thomas Aquinas (1225-1274), philosopher and theologian, a doctor (Lat. docere = to teach) of the Roman Catholic Church (known as 'the Angelic doctor' because of the sublimity of his writings and also as the 'Universal doctor', being the standard theologian for those studying for the Roman priesthood. According to Pope Benedict XV, "the Church has declared Thomas' doctrines to be her own". The 20th century witnessed a revival of "Thomism" in the RCC which continues to this day, with a branch known as "neo-Thomism" representing a synthesis of Thomas informed by modern philosophy.
** Cf Samuel Johnson's original definition of enthusiasm as "a vain belief of private revelation".
The text of the Summa, in the standard English translation of the English Dominicans, can be found on-line here: http://www.sacred-texts.com/chr/aquinas/summa/index.htm
The Commentary on the Sentences (of Peter Lombard) can be found here: http://dhspriory.org/thomas/Sentences.htm