From his marvelous Qui pluribus:
25. When ministers are ignorant or neglectful of their duty, then the morals of the people also immediately decline, Christian discipline grows slack, the practice of religion is dislodged and cast aside, and every vice and corruption is easily introduced into the Church. The word of God, which was uttered for the salvation of souls, is living, efficacious and more piercing than a two-edged sword. So that it may not prove to be unfruitful through the fault of its ministers, never cease, venerable brothers, from encouraging the preachers of this divine word to carry out most religiously the ministry of the Gospel. This should not be carried out by the persuasive words of human wisdom, nor by the profane seductive guise of empty and ambitious eloquence, but rather as a demonstration of the spirit and power.
26. Consequently, by presenting the word of truth properly and by preaching not themselves but Christ crucified, they should clearly proclaim in their preaching the tenets and precepts of our most holy religion in accordance with the teaching of the Catholic Church and the Fathers. They should explain precisely the particular duties of individuals, frighten them from vice, and inspire them with a love of piety. In this way the faithful will avoid all vices and pursue virtues, and so, will be able to escape eternal punishment and gain heavenly glory.