Catechism of the Council of Trent

"THAT the faithful people may approach to receive the sacraments with the greater reverence and devotion of mind, the holy Synod commands all bishops, not only when these shall have to be administered by themselves to the people, previously to explain their force and use in a manner suited to the capacity of the receivers, but also to endeavour that the same be piously and prudently observed by every publish-priest, even in the vernacular tongue, if need be, and it can be conveniently...
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"THAT the faithful people may approach to receive the sacraments with the greater reverence and devotion of mind, the holy Synod commands all bishops, not only when these shall have to be administered by themselves to the people, previously to explain their force and use in a manner suited to the capacity of the receivers, but also to endeavour that the same be piously and prudently observed by every publish-priest, even in the vernacular tongue, if need be, and it can be conveniently done, according to the form to be prescribed by the holy Synod for all the sacraments in a catechism, which bishops will take care to have faithfully translated into the vnlgar tongue, and expounded to the people by the parish-priests." Council of TrentPope Saint Pius X said in Acerbo Nimis: "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." And later on: "Since it is a fact that in these days adults need instruction no less than the young, all pastors and those having the care of souls shall explain the Catechism to the people in a plain and simple style adapted to the intelligence of their hearers. This shall be carried out on all holy days of obligation, at such time as is most convenient for the people, but not during the same hour when the children are instructed, and this instruction must be in addition to the usual homily on the Gospel which is delivered at the parochial Mass on Sundays and holy days. The catechetical instruction shall be based on the Catechism of the Council of Trent; and the matter is to be divided in such a way that in the space of four or five years, treatment will be given to the Apostles' Creed, the Sacraments, the Ten Commandments, the Lord's Prayer and the Precepts of the Church."