25 Venerable Brethren, We decree and command this by virtue of Our Apostolic Authority. It now rests with you to put it into prompt and complete execution in your respective dioceses, and by the power of your authority to see to it that these prescriptions of Ours be not neglected or, what amounts to the same thing, that they be not carried out carelessly or superficially. That this may be avoided, you must exhort and urge your pastors not to impart these instructions without having first prepared themselves in the work. Then they will not merely speak words of human wisdom, but "in simplicity and godly sincerity," imitating the example of Jesus Christ, Who, though He revealed "things hidden since the foundation of the world," yet spoke "all . . . things to the crowds in parables, and without parables . . . did not speak to them." We know that the Apostles, who were taught by the Lord, did the same; for of them Pope Saint Gregory wrote: "They took supreme care to preach to the uninstructed simple truths easy to understand, not things deep and difficult." In matters of religion, the majority of men in our times must be considered uninstructed.
Source: Acerbo Nimis (Vatican.va)