What kind of arguments did scholastics such as Thomas Aquinas use when talking about whether the Bible is true or not?
I looked through the table of contents of Aquinas’ Summa Contra Gentiles and his book Light of Faith and couldn’t find anything about that topic. I know that Justin Martyr and Irenaeus argued from the presence of true prophecy to prove that the Bible is God’s Word, and I think St. Cyril of Jerusalem does too.
Did they just accept scripture? They were only interested in commenting on scripture? Am I right?
I think the simplest thing to say is that St. Thomas believed that the Bible is true because it is revealed by God. We believe what the Bible says on the God’s authority.
The sole way to overcome an adversary of divine truth is from the authority of Scripture—an authority divinely confirmed by miracles. For that which is above the human reason we believe only because God has revealed it. (Summa Contra Gentiles I.9.2)
This doctrine is especially based upon arguments from authority, inasmuch as its principles are obtained by revelation: thus we ought to believe on the authority of those to whom the revelation has been made. Nor does this take away from the dignity of this doctrine, for although the argument from authority based on human reason is the weakest, yet the argument from authority based on divine revelation is the strongest. But sacred doctrine makes use even of human reason, not, indeed, to prove faith (for thereby the merit of faith would come to an end), but to make clear other things that are put forward in this doctrine. Since therefore grace does not destroy nature but perfects it, natural reason should minister to faith as the natural bent of the will ministers to charity. Hence the Apostle says: “Bringing into captivity every understanding unto the obedience of Christ” (2 Corinthians 10:5). Hence sacred doctrine makes use also of the authority of philosophers in those questions in which they were able to know the truth by natural reason, as Paul quotes a saying of Aratus: “As some also of your own poets said: For we are also His offspring” (Acts 17:28). Nevertheless, 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. Hence Augustine says (Epis. ad Hieron. xix, 1): “Only those books of Scripture which are called canonical have I learned to hold in such honor as to believe their authors have not erred in any way in writing them. But other authors I so read as not to deem everything in their works to be true, merely on account of their having so thought and written, whatever may have been their holiness and learning.” (Summa Theologiae I.1.8 ad 2)
You also ask whether St. Thomas accepted “just scripture” and he was “only interested in commenting on Scripture.” I am not absolutely sure what you mean by the first part of that question. If you mean that he accepted only scripture, it would depend on what you mean he accepted it as. Scripture is an infallible authority because it has the authority of divine revelation unlike the the writings of the individual Fathers, which have only a human authority (though a high degree of human authority), can err. However, if you mean to ask whether he is teaching the Protestant concept of sola scriptura, the answer is no. He does not deny the authority of the Church, and he teaches that the Church is infallible in her teaching. For instance,
The universal Church cannot err, since she is governed by the Holy Ghost, Who is the Spirit of truth: for such was Our Lord’s promise to His disciples (John 16:13): “When He, the Spirit of truth, is come, He will teach you all truth.” Now the symbol is published by the authority of the universal Church. Therefore it contains nothing defective. (ST III.1.9, sed contra)
And he says on the authority of the pope,
As stated above (Objection 1), a new edition of the symbol becomes necessary in order to set aside the errors that may arise. Consequently to publish a new edition of the symbol belongs to that authority which is empowered to decide matters of faith finally, so that they may be held by all with unshaken faith. Now this belongs to the authority of the Sovereign Pontiff, “to whom the more important and more difficult questions that arise in the Church are referred,” as stated in the Decretals [Dist. xvii, Can. 5. Hence our Lord said to Peter whom he made Sovereign Pontiff (Luke 22:32): “I have prayed for thee,” Peter, “that thy faith fail not, and thou, being once converted, confirm thy brethren.” The reason of this is that there should be but one faith of the whole Church, according to 1 Corinthians 1:10: “That you all speak the same thing, and that there be no schisms among you”: and this could not be secured unless any question of faith that may arise be decided by him who presides over the whole Church, so that the whole Church may hold firmly to his decision. Consequently it belongs to the sole authority of the Sovereign Pontiff to publish a new edition of the symbol, as do all other matters which concern the whole Church, such as to convoke a general council and so forth. (*ST II-II.1.10, co.)
And on whether there are things not contained in Scripture which must be believed,
The Apostles, led by the inward instinct of the Holy Ghost, handed down to the churches certain instructions which they did not put in writing, but which have been ordained, in accordance with the observance of the Church as practiced by the faithful as time went on. Wherefore the Apostle says (2 Thessalonians 2:14): “Stand fast; and hold the traditions which you have learned, whether by word”–that is by word of mouth–“or by our epistle”–that is by word put into writing. Among these traditions is the worship of Christ’s image. Wherefore it is said that Blessed Luke painted the image of Christ, which is in Rome. (ST III.25.3 ad 4)