Paraphrasing the Summa: First part, Question 1, Article 8

Question 1: The nature and extent of sacred doctrine.

Is Christian doctrine revealed by God?  Does it teach us about the supernatural as well as the natural?

Article 8. Whether sacred doctrine is a matter of argument?

Can Christian doctrine be justified and defended logically?  Are there logical parameters by which Christian doctrine can be explained?

Objection 1. It seems this doctrine is not a matter of argument. For Ambrose says (De Fide 1): "Put arguments aside where faith is sought." But in this doctrine, faith especially is sought: "But these things are written that you may believe" (John 20:31). Therefore sacred doctrine is not a matter of argument.

Matters of faith should not be disputed.  They are simply to be accepted without argument.  Since Christian doctrine is a matter of faith, it is not a matter of argument.

Objection 2. Further, if it is a matter of argument, the argument is either from authority or from reason. If it is from authority, it seems unbefitting its dignity, for the proof from authority is the weakest form of proof. But if it is from reason, this is unbefitting its end, because, according to Gregory (Hom. 26), "faith has no merit in those things of which human reason brings its own experience." Therefore sacred doctrineis not a matter of argument.

On the other hand, if Christian doctrine is not to be accepted without question, then it must have substantial reasons for one to believe.  These reasons, if they are forced upon us by authority, are not true reasons and do not amount to evidence.  

But, if they can be comprehended by man's unaided reason and substantiated with purely human evidence, then they are not truly from God.

But since it is claimed that Christian doctrine is from God, then it is not a matter of argument.

On the contrary, The Scripture says that a bishop should "embrace that faithful word which is according to doctrine, that he may be able to exhort in sound doctrine and to convince the gainsayers" (Titus 1:9).

St. Thomas disagrees.  The Sacred Word says that those in authority, in the Church, should be masters of Christian doctrine in order that they can convince others of the truth which is therein taught. 

I answer that, As other sciences do not argue in proof of their principles, but argue from their principles to demonstrate other truths in these sciences:

Sciences, in general, do not substantiate their principles.  But their principles are presupposed.  However, from their presupposed principles, they demonstrate other truths. 

so this doctrine does not argue in proof of its principles, which are the articles of faith, but from them it goes on to prove something else; as the Apostle from the resurrection of Christ argues in proof of the general resurrection (1 Corinthians 15).

It is the same with Christian doctrine.  For example, the resurrection of our Lord is a principle which demonstrates the truth of our own resurrection on the Last Day. 

However, it is to be borne in mind, in regard to the philosophical sciences, that the inferior sciences neither prove their principles nor dispute with those who deny them, but leave this to a higher science; 

There are certain philosophies which hold presuppositions which can't be proved at all nor can their proponents defend them against those who reject them.  This, they leave to more advanced sciences.   

whereas the highest of them, viz. metaphysics, can dispute with one who denies its principles, if only the opponent will make some concession; but if he concede nothing, it can have no dispute with him, though it can answer his objections.

Now, the highest of these philosophies, is metaphysics (i.e. the study of reality or existence) consists of that which studies all being.  This philosophy can defend its principles if the objector accepts at least some metaphysical teachings.  But if he accepts none of them, then, there is nothing to dispute.

Hence Sacred Scripture, since it has no science above itself,

Christian doctrine is the highest of all sciences. 

can dispute with one who denies its principles only if the opponent admits some at least of the truths obtained through divine revelation;

And can defend its principles against an opponent which admits some of that which God has revealed to man.

thus we can argue with heretics from texts in Holy Writ, and against those who deny one article of faith, we can argue from another. 

So, Christian doctrine can be defended and substantiated with anyone who accepts the validity of Scripture. 

If our opponent believes nothing of divine revelation, there is no longer any means of proving the articles of faith by reasoning,but only of answering his objections — if he has any — against faith.

But not with those who do not admit the existence of God or the validity, at least in part, of Christian doctrine.  All we can do with those is explain the reasons for our faith. 

Since faith rests upon infallible truth, and since the contrary of a truth can never be demonstrated, it is clear that the arguments brought against faith cannot be demonstrations, but are difficulties that can be answered.

God is truth.  Faith in God rests upon His existence.  Denials of the existence of God do not amount to proof that God does not exist.  Therefore, arguments against faith in God are of no substance.  But we can respond to these arguments.

Reply to Objection 1. Although arguments from human reason cannot avail to prove what must be received on faith, nevertheless, this doctrine argues from articles of faith to other truths.

The first objection says that articles of faith must be accepted without argument.

St. Thomas does not deny this objection.  Because human reason is not enough to prove that which can not be understood by the unaided human mind.  None the less, Christian doctrine should be taught to all men in order that they might also come to faith.

Reply to Objection 2. This doctrine is especially based upon arguments from authority, inasmuch as its principles are obtained by revelation:

Christian doctrine comes from the very highest authority, God.  And it is passed down by those whom He appointed in authority to do so.

thus we ought to believe on the authority of those to whom the revelation has been made.

Thus, it is true that we ought to accept and believe without disputing. 

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.

Although an "I say so" argument is of the weakest kind when it is made by a human.  Yet, God's say so (i.e. God's word) is the highest of arguments because it is based upon the greatest of wisdom. 

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.

Although unaided human reason can not grasp all articles of faith.  Yet, human logic can be used to clarify some of the truths taught in Christian 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.

God's love does not destroy man, but makes him more perfect.  Therefore, man should seek to attain faith as much as he seeks to attain love.

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).

And, since faith is greater than human reason.  Then, human reason should be subordinate to faith and should obey the principles of faith.  Therefore, Christian doctrine uses all the truths discovered by natural reason and explained by human philosophers to support the truths revealed by God.  Because, as Scripture says, we are all God's children. 

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.

But Christian doctrine uses these proofs merely to support God's truths.  Not to prove them.  The only indisputable evidence we have to prove God's truth is from the Sacred Scriptures.  And the evidence presented by the Early Church Fathers can be used further support, but it is not indisputable.

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."

Therefore, it is upon Sacred Scripture that we rely to prove the truth of Christian doctrine.  Not on any other books, no matter how holy the authors may be. 

*  Note:  Don't let this confuse you.  St. Thomas is not teaching Sola Scriptura.  Note that he refers to St. Augustine in the end of his explanation.  St. Thomas relies heavily upon St. Augustine's explanations.  And St. Augustine is famous for saying, "I would not accept even the Gospel, if not for the authority of the Catholic Church" (against manichaeus).

Here's more proof.  If you look at his "I answer that" section, where he speaks of "heretics".  The very word, "heretic" implies an authority which has the power to declare someone a heretic based upon their erroneous understanding of Christian doctrine.