7 It is necessary that Christian wisdom, which abounds in a wonderful light, should shine before the eyes of all, so that the darkness of ignorance, which is the greatest enemy to religion, having been dispelled, the truth may shine forth far and wide, and happily reign. Nay more, it behoves that those manifold errors be refuted and dispelled which, taking their rise either in ignorance or wickedness or prejudiced opinions, perversely call away the minds of men from Catholic truth, and engender a certain hatred of it in their dispositions. This great duty, which is "to exhort in sound doctrine and to convince the gainsayers" ( Ep. Tit . i., 9), belongs to the order of priests, who hold it legitimately, imposed by Christ our Lord when He sent them forth to teach all nations, by His divine power, "going into the whole world preach the gospel to every creature" ( Mar . xvi. 15), equally plainly as the bishops, chosen in place of the apostles, are set over the Church of God, the priests are their assistants. If ever these duties have been fully and perfectly carried out it was in the first ages of our religion and in the following centuries during that great struggle with heathen tyranny which raged for so long a time, whence the priestly band and the most holy order of Fathers and Doctors whose wisdom and eloquence will be ever held in memory and admiration, obtained their great glory. For indeed Christian doctrine deeply treated of by them, fully explained, and most valiantly maintained, by that means spread forth the more its truth and divine excellence. On the other hand appeared the doctrine of the heathens, confuted and despised even by the unlearned, as having no consistency, full of absurdities, useless. But in vain did the adversaries try to arrest and stop that course of Catholic wisdom; in vain did they seek objections from the schools of Greek philosophy, especially from those of Plato and Aristotle, with high-sounding words indeed. For our champions, declining not even that kind of contest, applied themselves to the learning and study of the heathen philosophers; having examined with the greatest diligence what each one of them had professed, they took these things into consideration one by one; they examined them, they compared them; many things were rejected or corrected by them; not a few were justly approved of and accepted; they also discovered and established by them, that those things which are proved to be false by human reason and intelligence, are in the same manner opposed to Christian doctrine, so that he who withstands and opposes this doctrine, of necessity equally withstands and opposes reason. Contests of this kind were entered into by our fathers, and splendid victories obtained, and these were achieved, not only by the virtue and arms of faith, but also by the aid of human reason; which indeed, guided by the light of divine wisdom, entered boldly upon the path of truth, from ignorance of many things, and as it were out of a forest of errors. This admirable agreement and consent of the faith with reason, although it has been honoured by the learned works of many, yet as it were built up in one edifice and shown at one view, shines forth especially in that work of St. Augustine, De Civitate Dei , and equally in the Summa of St. Thomas Aquinas, in which books, indeed, are contained whatever things were deeply thought out and considered by wise men, and in them we may seek for the beginnings and fount of that eminent school of learning called Christian theology. The memory of such illustrious examples should be remembered and cherished by the clergy, since in many ways ancient weapons are being sharpened by our adversaries, and nearly the same old battles are to be re-fought. Thus only the heathen formerly objected to the Christian religion, that they should not be led away from the ancient and accustomed rites of their divinities, but now the most iniquitous endeavour of wicked men contend that they should eradicate from Christian people all divine and most necessary teaching connected with Our holy faith, and that they may use them worse than the heathen, and may involve them in the greatest misery, namely, the subversion and contempt of all faith and religion. Of which impure plague, than which none is more detestable, those were the founders who attributed to man that by the light of nature each one could know and judge concerning doctrine divinely revealed by virtue of his own reason and judgment, and that there was no necessity to submit to the authority of the Church and the Roman Pontiff, whose sole right it is, by divine command and appointment to be the guardian of that doctrine, to hand it on and to judge truly concerning it. Thence the way easily opened, though to them it lay open most miserably, for denying and discarding all things and the powers of man: then insolently denying that there was any authority which emanated from God or even that there was a God, they at length lapsed into absurd theories of Idealism and Materialism. But this prostitution of the highest things, those who are named Rationalists or Naturalists do not hesitate to call by the false name of scientific and social progress, which in truth is nothing less than the destruction and ruin of both.
Source: Officio Sanctissimo (Vatican.va)