18 To express briefly, therefore, what We have just said, let Us quote these words of St. Peter Damian: "For the preacher two things are especially necessary: namely that his words should be rich in ghostly wisdom, and that his life should be conspicuous for the luster of its piety. But if a priest is unequal to being both holy in life and rich in learning, holiness of life is, without question, to be preferred to mere learning. For the example of a saintly life is more powerful than eloquence and a studied delivery.... The priest who discharges the office of preaching should cause showers of heavenly wisdom to fall from his lips, and from his life rays of piety to shine out, just as the angel in telling the shepherds of Our Lord's birth, both shone with great splendor and expressed in words the tidings he had come to announce." [Epp. Lib. i, Ep. i ad Cinthium Urbis Praef.]
Source: Humani Generis Redemptionem (Vatican.va)