15 The chief elements of this duty consist in professing openly and unflinchingly the Catholic doctrine, and in propagating it to the utmost of our power. For, as is often said, with the greatest truth, there is nothing so hurtful to Christian wisdom as that it should not be known, since it possesses, when loyally received, inherent power to drive away error. So soon as Catholic truth is apprehended by a simple and unprejudiced soul, reason yields assent. Now, faith, as a virtue, is a great boon of divine grace and goodness; nevertheless, the objects themselves to which faith is to be applied are scarcely known in any other way than through the hearing. "How shall they believe Him of whom they have not heard? and how shall they hear without a preacher? Faith then cometh by hearing, and hearing by the word of Christ."(14) Since, then, faith is necessary for salvation, it follows that the word of Christ must tie preached. The office, indeed, of preaching, that is, of teaching, lies by divine right in the province of the pastors, namely, of the bishops whom "the Holy Spirit has placed to rule the Church of God."(15) It belongs, above all, to the Roman Pontiff, vicar of Jesus Christ, established as head of the universal Church, teacher of all :hat pertains to morals and faith.
Source: Sapientiae Christianae (Vatican.va)