13 For, in the first place, as the Council of Trent teaches, Jesus Christ at the Last Supper instituted the sacrifice and the priesthood of the New Covenant: "our Lord and God, although once and for all, by means of His death on the altar of the cross, He was to offer Himself to God the Father, that thereon He might accomplish eternal Redemption; yet because death was not to put an end to his priesthood, at the Last Supper, the same night in which He was betrayed in order to leave to His beloved spouse the Church, a sacrifice which should be visible (as the nature of man requires), which should represent that bloody sacrifice, once and for all to be completed on the cross, which should perpetuate His memory to the end of time, and which should apply its saving power unto the remission of sins we daily commit, showing Himself made a priest forever according to the order of Melchisedech, offered to God the Father, under the appearance of bread and wine, His Body and Blood, giving them to the apostles (whom He was then making priests of the New Covenant) to be consumed under the signs of these same things, and commanded the Apostles and their successors in the priesthood to offer them, by the words 'Do this in commemoration of Me.' "
Source: Ad Catholici Sacerdotii (Vatican.va)