28 just as Moses made the Old Testament sacred with the blood of calves, (13) so too Christ the Lord took the New Testament, of which He is the Mediator, and made it sacred through His own blood, in instituting the mystery of the Eucharist. For, as the Evangelists narrate, at the Last Supper "he took bread, and blessed and broke it, and gave it to them, saying, This is my body, given for you; do this for a commemoration of me. And so with the cup, when supper was ended, This cup, he said, is the new testament, in my Blood which is to be shed for you." (l4) And by bidding the Apostles to do this in memory of Him, He made clear that He wanted it to be forever repeated. This intention of Christ was faithfully carried out by the primitive Church through her adherence to the teaching of the Apostles and through her gatherings to celebrate the Eucharistic Sacrifice. As St. Luke is careful to point out, "They occupied themselves continually with the Apostles' teaching, their fellowship in the breaking of bread, and the fixed times of prayer." (l5) The faithful used to derive such spiritual fervor from this practice that it was said of them that "there was one heart and soul in all the company of the believers." (16)
Source: Mysterium Fidei (Vatican.va)