1 The office divinely committed to Us of feeding the Lord's flock has especially this duty assigned to it by Christ, namely, to guard with the greatest vigilance the deposit of the faith delivered to the saints, rejecting the profane novelties of words and oppositions of knowledge falsely so called. There has never been a time when this watchfulness of the supreme pastor was not necessary to the Catholic body; for, owing to the efforts of the enemy of the human race, there have never been lacking "men speaking perverse things" ( Acts xx. 30), "vain talkers and seducers" ( Tit . i. 10), "erring and driving into error" (2 Tim . iii. 13). Still it must be confessed that the number of the enemies of the cross of Christ has in these last days increased exceedingly, who are striving, by arts, entirely new and full of subtlety, to destroy the vital energy of the Church, and, if they can, to overthrow utterly Christ's kingdom itself. Wherefore We may no longer be silent, lest We should seem to fail in Our most sacred duty, and lest the kindness that, in the hope of wiser counsels, We have hitherto shown them, should be attributed to forgetfulness of Our office.
Source: Pascendi Dominici Gregis (Vatican.va)