8 On July 31, 1894, Our Predecessor, through the Sacred Congregation of Bishops and Regulars, called the Bishops' attention to this very serious problem. The regulations and norms set up in that Pontifical document We now affirm and renew, commanding the Bishops to form their conscience according to it, lest the words of the Prophet Nahum might be applied to any of them: "Thy shepherds have slumbered." No one can have the faculty of preaching "unless he first be approved of in life, knowledge and morals." Priests of other dioceses should not be allowed to preach unless they have testimonial letters from their own Bishop. Let the subject of their sermons be that which the Divine Savior indicated when He said: "Preach the gospel . . . teaching them to observe all that I commanded you." Or, according to the Council of Trent, "announcing to them the vices they should avoid and the virtues they should follow in order to escape eternal punishment and attain heavenly glory." Therefore, let those arguments better suited to journalistic campaigns and lecture halls be completely banished from the holy place. Let moral preaching be preferred to sermons which are, to say the least, fruitless. Let the preacher speak "not in the persuasive words of wisdom, but in the demonstration of the Spirit and of power." The principal source, therefore, from which preaching will derive its strength will be the Sacred Scriptures, understood not according to the private judgment of minds very frequently blinded by passions, but according to the traditions of the Church and the interpretations of the holy Fathers and Councils.
Source: Pieni L'Animo (Vatican.va)