53 Hence too he condemns, here or elsewhere, the perverse opinion of those who conceded too much to philosophy by attributing to it the right to invade the domain of theology. In refuting this foolish theory he defines well the confines proper to each, and hints sufficiently clearly at the functions of reason in the things of divinely revealed doctrine: "Our faith," he says, "must be defended by reason against the impious" (In lib . ii. Epist. S. Anselmi , ep. 41). But how and how far? The question is answered in the words that follow: "It must be shown to them reasonably how unreasonable is their contempt of us" ( Ibid .). The chief office, therefore, of philosophy is to show us the reasonableness of our faith and the consequent obligation of believing the divine authority proposing to us the profoundest mysteries, which with all signs of credibility that testify to them, are supremely worthy of being believed. Far different is the proper function of Christian theology, which is based on the fact of divine revelation and renders more solid in the faith those who already profess to enjoy the honor of the name of Christian. "Hence it is altogether clear that no Christian should dispute as to how that is not which the Catholic Church believes with the heart and confesses with the mouth, but even holding beyond all doubt the same faith, loving and living according to it, must seek as far as reason is able, how it is. If he is able to understand let him return thanks, let him not prepare his horns for attack, but bow his head in reverence" ("De fide Trinitatis," cap 2).
Source: Communium Rerum (Vatican.va)