4 In view, therefore, of the gravity of the subject, alive to the responsibilities of the apostolic office and warned by them, animated, moreover, by the example of Our two immediate Predecessors, We realize that it must be Our earnest endeavor everywhere to bring back the preaching of the Word of God to the norm and ideal to which it must be directed according to the command of Christ Our Lord, and the laws of the Church. In the first place, Venerable Brethren, We must look for the causes of our deviations from the right path in this matter. They may be reduced to three: for either the one chosen to preach is not the right person, or his office is not performed with the right intention, or in the right way.
Source: Humani Generis Redemptionem (Vatican.va)