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St. Thomas was a master of understanding limitations. He understood his limitations when it came to the powers of his intellect and stopped accordingly. In his work De Rationibus Fidei ad Cantorem Antiochenum, he lays out the limitations of the apologetic task with sublime simplicity and prudence.
He begins by extolling the glories of the faith. By "the faith," we are to understand those objects of Divine Revelation that we are to believe in order to be saved. For, it is by this faith that "Blessed Peter the apostle received a promise from the Lord." Further, this faith is so strong that "hold out inviolate against these gates of hell."
It is from this point of view that we are to aproach defending the faith. It is something with a sure strength that we can trust will not be the slightest impugned by all of the forces of hell. It is something that will easily break to peices all things placed against it, as Our Lord does, so also will the faith "break to pieces" all falsehood. In this hope, "we can be safe against any attacks or ridicule of unbelievers against our faith."
He then demarkates those things in which Christian faith and hope principally consists. The Christian faith principally consists in "acknowledging the holy Trinity, and it specially glories in the cross of our Lord Jesus Christ." Christian hope consists of two things, first, in grace, and, second, in glory. This is where we are to focus our defence of the faith.
Third, in the second chapter of the work, he gives us a guide on how to argue with unbelievers. First, he gives this thesis, i.e., "in disputations with unbelievers about articles of the faith, you should not try to prove the faith by necessary reasons." By, "articles of the faith" he is speaking of those things which are only acessible by special revelation, for example, the Trinity, the Incarnation, and the like. By "necessary reasons," he is not saying that we shouldn't show probable reasons for certain things, e.g., the incarnation is fitting in order to bring us closer to God, rather, he is saying that we should not overplay our hand by saying that these arguments prove the truth definitively. Rather, these merely show the fittingness of the articles of faith.
Then, he gives a reason for the thesis, "This would belittle the sublimity of the faith, whose truth exceeds not only human minds but also those of angels; we believe in them only because they are revealed by God." In this, there are three parts, the third is the most important for our current context.
First, he proves it from the "sublimity of the faith." In Catholic theology, we distinguish between that which is natural and that which is supernatural, above all created nature, actual and possible. Now, such things as the hypostatic union, the beatific vision, sanctifying grace, and the like are strictly supernatural. Thus, to have them as necessary conclusions based on natural premises would be to belittle supernatural truths to the level of natural truths. For, it is a fundamental principle of logic that the conclusion is contained in the premises. Now, a supernatural conclusion cannot be contained in natural premises. Thus, making such an argument would imply that it is not supernatural, but natural.
Second, he proves it from the littleness of our minds. For, our minds know things through sense perception and discursive reasoning. Now, the latter has already been shown to be impotent in discovering this mystery. Further, the former would be equally laughable.
Third, he proves it from the nature of faith. This is the most prophetic to our context. We are in a context where most cannot distinguish between being convinced that it is raining outside and being convinced of the truths of the faith. For, faith is the supernatural ascent of the mind to the articles of the faith for the very fact that God revealed them. Being convinced of the historical truth of the gospel account is not faith. Being convinced of the existence of God is not faith. Faith is not something which can be conjured up by any human efforts. Rather, it is strictly an infused habit. Arguments for the faith serve, not to bring about faith, but to remove certain impediments.
Then, he gives a corallary, "Yet whatever come from the supreme truth cannot be false, and what is not false cannot be repudiated by any necessary reason. Just as our faith cannot be proved by necessary reasons, because it exceeds the human mind, so because of its truth it cannot be refuted by any necessary reason."
He then draws the opposing conclusion, i.e., as a supernatural truth, it cannot be disproved by natural reason. For, to disprove something is to show some contradiction with a necessarily established truth. Now, necessarily established truths are discerned by reason. Thus, due to the limitations of our minds, as outlined above, those necessarily established truths are natural. Now, what is supernatural is not virtually contained in what is natural, as outlined above. Thus, not only does what is supernatural not suffer necessary demonstration, so also does it not suffer necessary refutation.
Lastly, St. Thomas draws the practical conclusion, "So any Christian disputing about the articles of the faith should not try to prove the faith, but defend the faith...so that reason can show that what the Catholic faith holds is not false."
In our apologetics, rather than giving positive, necessary, demonstrative proofs for the faith, rather, we declare the truth of the faith and defend it from all arguments against it.