In De Rationibus Fidei, St. Thomas explains how best to go about arguing with those who do not identify as Christians:
First of all I wish to warn you that in disputations with unbelievers about articles of the Faith, you should not try to prove the Faith by necessary reasons. 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.
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. So any Christian disputing about the articles of the Faith should not try to prove the Faith, but defend the Faith. Thus blessed Peter (1 Pet 3:15) did not say: “Always have your proof”, but “your answer ready,” so that reason can show that what the Catholic Faith holds is not false.
Aquinas means some very specific things by “articles of faith.” For instance, God’s existence for him was a matter of rational demonstration. But the Trinity or the Atonement were matters of “the Faith” meaning that they were revealed by God and not things which could have been determined by mere investigation or deduction from first principles. Aquinas doesn’t mean, “some things you just take on faith [belief for no reason].” He means that certain articles of the faith aren’t to be proved in discussing Christianity with those who do not adhere to it, but rather to be defended against charges of falsehood. Far from being baptized Aristotle, Aquinas here claims that the revelation of God, though perfectly reasonable, is within the purview of reason to be examined once revealed though not within the purview of reason to be proven or discovered.