36 Moreover, the priest must teach the truths of faith; but the truths of religion are never so worthily and effectively taught as when taught by virtue; because in the common saying: "Deeds speak louder than words." The priest must preach the law of the Gospel; but for that preaching to be effective, the most obvious and, by the Grace of God, the most persuasive argument, is to see the actual practice of the law in him who preaches it. St. Gregory the Great gives the reason: "The voice which penetrates the hearts of the hearers, is the voice commended by the speaker's own life; because what his word enjoins, his example helps to bring about." This exactly is what Holy Scripture says of our Divine Savior: He "began to do and to teach." And the crowds hailed Him, not so much because "never did man speak like this man," but rather because "He hath done all things well." On the other hand, they who "say and do not," practicing not what they preach, become like the scribes and Pharisees. And Our Lord's rebuke to the other hand, they who "say and do not," practicing not what they preach, the word of God, was yet administered publicly, in the presence of the listening crowd: "The Scribes and Pharisees have sitten on the chair of Moses. All things therefore whatsoever they shall say to you observe and do: but according to their work do ye not." A preacher who does not try to ratify by his life's example the truth he preaches, only pulls down with one hand what he builds up with the other. On the contrary, God greatly blesses the labor of those heralds of the gospel who attend first to their own holiness; they see their apostolate flourishing and fruitful, and in the day of the harvest, "coming they shall come with joyfulness carrying in their sheaves."
Source: Ad Catholici Sacerdotii (Vatican.va)