10 Both these purposes therefore must be carried out by the men who devote themselves to the sacred ministry of preaching. They must diffuse the light of truth made known by God, and in those who hear them they must quicken and nourish the supernatural life. In a word, by seeking the salvation of souls they are to promote the glory of God. As it would, therefore, be wrong to call anyone a doctor who does not practice medicine, or to style anyone a professor of some art who does not teach that art, he who in his preaching neglects to lead men to a fuller knowledge of God and on the way of eternal salvation may be called an idle declaimer, but not a preacher of the Gospel. And would there were no such declaimers! What motive is it that sways them mostly. Some are moved by the desire of vain-glory and to satisfy it: "They ponder how they can express high rather than practical thoughts, causing weak minds to admire them, instead of working out the salvation of their hearers. They are ashamed of what is simple and plain, lest they be thought to know nothing else. They are ashamed to give milk to the little ones. [Gillebertus Abb. In Cant. Canticor . Serm xxvii, 2.] Whereas Jesus Christ proved by the lowliness of his hearers that He was the One whom men were awaiting: "The poor have the Gospel preached to them." [ Matt . xi:5] What efforts do such men make to acquire reputation by their sermons from the size and wealth of the cities and splendor of the great churches in which they preach? But since among the truths revealed by God there are some which frighten the weakness of our corrupt nature, and which therefore are not calculated to attract the multitude, they carefully avoid them, and treat themes, in which, the place accepted, there is nothing sacred. Not seldom it happens that in the very midst of a discourse upon the things of eternity, they turn to politics, particularly if any questions of this kind just then deeply engross the minds of their hearers. They seem to have only one aim, to please their hearers and curry favor with those whom St. Paul describes as "having itching ears." [II Tim . vi:3] Hence that unrestrained and undignified gesture such as may be seen on the stage or on the hustings, that effeminate lowering of the voice or those tragic outbursts; that diction peculiar to journalism; those frequent allusions to profane and non-Catholic literature, but not to the Sacred Scriptures or the Holy Fathers; finally that volubility of utterance often affected by them, wherewith they strike the ears and gain their hearers' admiration, but give them no lesson to carry home. How sadly are those preachers deceived! Granted that they receive the applause of the uneducated, which they seek with such great favor, and not without sacrilege, is it really worth while when we consider that they are condemned by every prudent man, and, what is worse, have reason to fear the stern judgment of Christ?
Source: Humani Generis Redemptionem (Vatican.va)