On 15th April, 1905, Pope St. Pius X issued his Encyclical Letter Acerbo Nimis on the teaching Christian Doctrine:
8. We do not think it necessary to set forth here the praises of such instruction or to point out how meritorious it is in God's sight. If, assuredly, the alms with which we relieve the needs of the poor are highly praised by the Lord, how much more precious in His eyes, then, will be the zeal and labor expended in teaching and admonishing, by which we provide not for the passing needs of the body but for the eternal profit of the soul! Nothing, surely, is more desirable, nothing more acceptable to Jesus Christ, the Savior of souls, Who testifies of Himself through Isaias: "To bring good news to the poor he has sent me."
9. Here then it is well to emphasize and insist that for a priest there is no duty more grave or obligation more binding than this. Who, indeed, will deny that knowledge should be joined to holiness of life in the priest? "For the lips of the priest shall keep knowledge." The Church demands this knowledge of those who are to be ordained to the priesthood. Why? Because the Christian people expect from them knowledge of the divine law, and it was for that end that they were sent by God. "And they shall seek the law at his mouth; because he is the angel of the Lord of hosts." Thus the bishop speaking to the candidates for the priesthood in the ordination ceremony says: "Let your teaching be a spiritual remedy for God's people; may they be worthy fellow-workers of our order; and thus meditating day and night on His law, they may believe what they read, and teach what they shall believe."
10. If what We have just said is applicable to all priests, does it not apply with much greater force to those who possess the title and the authority of parish priests, and who, by virtue of their rank and in a sense by virtue of a contract, hold the office of pastors of souls? These are, to a certain extent, the pastors and teachers appointed by Christ in order that the faithful might not be as "children, tossed to and fro and carried about by every wind of doctrine devised in the wickedness of men," but that practicing "the truth in love," they may, "grow up in all things in him who is the head, Christ."
11. For this reason the Council of Trent, treating of the duties of pastors of souls, decreed that their first and most important work is the instruction of the faithful. It therefore prescribes that they shall teach the truths of religion on Sundays and on the more solemn feast days; moreover during the holy seasons of Advent and Lent they are to give such instruction every day or at least three times a week. This, however, was not considered enough. The Council provided for the instruction of youth by adding that the pastors, either personally or through others, must explain the truths of religion at least on Sundays and feast days to the children of the parish, and inculcate obedience to God and to their parents. When the Sacraments are to be administered, it enjoins upon pastors the duty to explain their efficacy in plain and simple language.
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.