19 However, to return to St. Paul, if we ask on what subjects he was wont to discourse when he preached, he condenses them all in these words: "For I judged not myself to know anything among you but Jesus Christ and Him crucified." [I Cor . ii:2] To make men know Jesus Christ better and better, and to make that knowledge have a bearing, moreover, not only on their faith, but on their lives as well, was the object of that apostolic man's every endeavor. This was the object of every throb of his apostolic heart. Therefore all Christ's doctrines and commands, even the sterner ones, were so proclaimed by St. Paul that he did not restrict, gloss over or tone down what Christ taught regarding humility, self-denial, chastity, contempt of the world, obedience, forgiveness of enemies, and the like, nor was he afraid to tell his hearers that they had to make a choice between the service of God and the service of Belial, for they could not serve both, that when they leave this world, a dread judgment awaits them; that they cannot bargain with God; they may hope for life everlasting if they keep His entire law, but if they neglect their duty and indulge their passions, they will have nothing to expect but eternal fire. For our "Preacher of truth" never imagined that he should avoid such subjects, because, owing to the corruption of the age, they appeared too stern to his hearers. Therefore it is clear how unworthy of commendation are those preachers who are afraid to touch upon certain points of Christian doctrine lest they should give their hearers offense. Does a physician prescribe useless remedies to his patient, merely because the sick man rejects effective ones? The test of the orator's power and skill is his success in making his hearers accept the stern truth he is preaching. How did the Apostle unfold the subjects of which he treated? "Not in the persuasive words of human wisdom." [I Cor . ii:4] It is perfectly plain, Venerable Brethren, how important for everybody it is that they should thoroughly realize this, since we see that not a few of our sacred preachers overlook in their sermons the Sacred Scriptures, the Fathers and Doctors of the Church," and the arguments based on sacred theology, and for the most part, make their appeals only to reason. Unquestionably that is wrong, for in the supernatural order, merely human resources are of no help whatever. But the objection may be urged: The people have no confidence in the preacher who insists on Divinely revealed truths. Is that true? With non-Catholics, granted. However, when the Greeks sought the Wisdom, forsooth, of this world, the Apostle, nevertheless, preached to them Christ crucified. If we direct our attention, however, to Catholic people, even those men among them who are unfriendly to us, generally keep in their hearts the roots of faith. Their intellects are blinded because their souls are corrupted. Lastly, what end did St. Paul have in his preaching? Not to please men, but Christ. "If I yet pleased men, I should not be the servant of Christ." [ Gal. i: 10] As his heart was on fire with the love of Christ, he sought for nothing save the glory of Christ. O that all are engaged in the ministry of the Word were true lovers of Jesus Christ. Would that all could repeat these words of St. Paul: "For whom Jesus Christ] I have suffered the loss of all things," [ Phil . iii:8] and "To me to live is Christ." [ Phil. i:21] Only those who glow with love themselves know how to set on fire the hearts of others. Wherefore St. Bernard gave a preacher this counsel: "If you are wise, be a reservoir, not a conduit, be full yourself of what you preach and do not think it enough to pour it out for others." [ In Cant. Serm . 18] The Doctor then adds: "Today we have in the Church a profusion of conduits, but how few are the reservoirs!"
Source: Humani Generis Redemptionem (Vatican.va)