12 These prescriptions of the Council of Trent have been summarized and still more clearly defined by Our Predecessor, Benedict XIV, in his Constitution Esti minime . "Two chief obligations," he wrote, "have been imposed by the Council of Trent on those who have the care of souls: first, that of preaching the things of God to the people on the feast days; and second, that of teaching the rudiments of faith and of the divine law to the youth and others who need such instruction." Here the wise Pontiff rightly distinguishes between these two duties: one is what is commonly known as the explanation of the Gospel and the other is the teaching of Christian doctrine. Perhaps there are some who, wishing to lessen their labors, would believe that the homily on the Gospel can take the place of catechetical instruction. But for one who reflects a moment, such is obviously impossible. The sermon on the holy Gospel is addressed to those who should have already received knowledge of the elements of faith. It is, so to speak, bread broken for adults. Catechetical instruction, on the other hand, is that milk which the Apostle Peter wished the faithful to desire in all simplicity like newborn babes.
Source: Acerbo Nimis (Vatican.va)