6 But now, Venerable Brethren, the custom seems to be far different. Among our sacred orators, there are too many to whom might well be applied that complaint which the Lord makes through the Prophet Jeremias: "I did not send prophets, yet they ran." ( Jer . xxiii:21] For the man who owing to his peculiar bent of mind, or any other cause, should choose to undertake the ministry of the Word, finds easy access to the pulpits of our churches as to a drill-ground where any one may practice at will. Therefore, Venerable Brethren, it is your duty to see that such a grave abuse should disappear, and since you will have to render to God and to His Church an account of the manner in which you feed your flock, allow no one to creep unbidden into the sheepfold and to feed the sheep of Christ according to his fancy. Therefore let no one henceforth preach in your dioceses except on your summons and with your approval. Here therefore we would have you pay the greatest heed to whom you commit so sacred a duty. By the decree of the Council of Trent Bishops are permitted to select for this office those only who are "fit," i.e. those who "can exercise the ministry of preaching with profit to souls."
Source: Humani Generis Redemptionem (Vatican.va)