38 Above all things, remember, beloved sons, that an indispensable condition of true zeal and the best pledge of success in the works to which hierarchical obedience consecrates you is purity and holiness of life. "Jesus began by practicing before preaching."(28) Like Him, the priest must preface preaching by word by preaching by example. "Separated from the world and its concerns (say the Fathers of the Council of Trent), clerics have been placed on a height where they are visible and the faithful look into their lives as into a mirror to know what they are to imitate. Hence clerics and all they whom God has called specially to His service should so regulate their actions and morals that there may be nothing in their deportment, manners, movements, words and in all the other details of their life which is not deeply impressed with religion. They must carefully avoid faults which, though trivial, in others would be very serious to them, in order that there be not a single one of their acts which does not inspire respect in all."(29) With these recommendations of the sacred Council, which we would wish, beloved sons, to engrave in all your hearts, those priests who certainly fail to comply, who adopted in their preaching language our of harmony with the dignity of their priesthood and the sacredness of the word of God; who attended popular meetings where their presence could only excite the passions of the wicked and of the enemies of the Church, and who exposed themselves to the grossest insults without profit to any one, and to the astonishment, if not scandal, of the pious faithful; who assumed the habits, manners, conduct and spirit of laymen. Salt must certainly be mingled with the mass which it is to preserve from corruption, but it must at the same time defend itself against the mass under pain of losing all savor and becoming of no use except to be thrown out and trampled under foot.(30)
Source: Depuis le Jour (Vatican.va)