21 Since they attack the very root of faith either by openly denying, hypocritically undermining, or misrepresenting revealed doctrine, we should above all recall the truth Charles often taught. "The primary and most important duty of pastors is to guard everything pertaining to the integral and inviolate maintenance of the Catholic Faith, the faith which the Holy Roman Church professes and teaches, without which it is impossible to please God." Again: "In this matter no diligence can be too great to fulfill the certain demands of our office." We must therefore use sound doctrine to withstand "the leaven of heretical depravity," which if not repressed, will corrupt the whole. That is to say, we must oppose these erroneous opinions now deceitfully being scattered abroad, which, when taken all together, are called Modernism . With Charles we must be mindful "of the supreme zeal and excelling diligence which the bishop must exercise in combating the crime of heresy."
Source: Editae Saepe (Vatican.va)