8 To make your task easier in this matter, Venerable Brethren, We desire that hereafter severe judgment be passed on these two points: on the character, namely, and learning of those who seek to obtain authority to preach, just as is done on the character and learning of those priests, who would hear confessions. Whoever, therefore, is found defective in either regard must without any consideration whatever be debarred from a function for which he is not qualified. Your dignity demands this, since, as We have said, the preachers are your substitutes. The good of Holy Church demands it, for surely if any one should be the "salt of the earth and the light of the world," [ Matt . v: 13, 14], it is the man who is engaged in the ministry of the Word. With these rules duly laid down it may seem superfluous to proceed further and explain what should be the purpose and method of the sacred function of preaching. For if we select our sacred orators in accordance with the norm given by Us, they cannot fail to be endowed with the requisite virtues, or set before themselves a worthy purpose or follow the right method in their preaching. Nevertheless it will be serviceable to throw some light on these two points, that thus the reason may become clearer why some fall short of the ideal of a good preacher.
Source: Humani Generis Redemptionem (Vatican.va)