9 We know that there are some who, in their overestimate of the human faculties, maintain that as soon as man's intellect becomes subject to divine authority it falls from its native dignity, and hampered by the yoke of this species of slavery, is much retarded and hindered in its progress toward the supreme truth and excellence. Such an idea is most false and deceptive, and its sole tendency is to induce foolish and ungrateful men wilfully to repudiate the most sublime truths, and reject the divine gift of faith, from which the fountains of all good things flow out upon civil society. For the human mind, being confined within certain limits, and those narrow enough, is exposed to many errors and is ignorant of many things; whereas the Christian faith, reposing on the authority of God, is the unfailing mistress of truth, whom whoso followeth he will be neither enmeshed in the snares of error nor tossed hither and thither on the waves of fluctuating opinion. Those, therefore, who to the study of philosophy unite obedience to the Christian faith, are philosophizing in the best possible way; for the splendor of the divine truths, received into the mind, helps the understanding, and not only detracts in nowise from its dignity, but adds greatly to its nobility, keenness, and stability. For surely that is a worthy and most useful exercise of reason when men give their minds to disproving those things which are repugnant to faith and proving the things which conform to faith. In the first case they cut the ground from under the feet of error and expose the viciousness of the arguments on which error rests; while in the second case they make themselves masters of weighty reasons for the sound demonstration of truth and the satisfactory instruction of any reasonable person. Whoever denies that such study and practice tend to add to the resources and expand the faculties of the mind must necessarily and absurdly hold that the mind gains nothing from discriminating between the true and the false. Justly, therefore, does the Vatican Council commemorate in these words the great benefits which faith has conferred upon reason: Faith frees and saves reason from error, and endows it with manifold knowledge.(26) A wise man, therefore, would not accuse faith and look upon it as opposed to reason and natural truths, but would rather offer heartfelt thanks to God, and sincerely rejoice that, in the density of ignorance and in the flood-tide of error, holy faith, like a friendly star, shines down upon his path and points out to him the fair gate of truth beyond all danger of wandering.
Source: Aeterni Patris (Vatican.va)