21 The Second Vatican Council teaches that the celebration of the Eucharist is at the centre of the process of the Church's growth. After stating that “the Church, as the Kingdom of Christ already present in mystery, grows visibly in the world through the power of God”, then, as if in answer to the question: “How does the Church grow?”, the Council adds: “as often as the sacrifice of the Cross by which 'Christ our pasch is sacrificed' ( 1 Cor 5:7) is celebrated on the altar, the work of our redemption is carried out. At the same time in the sacrament of the Eucharistic bread, the unity of the faithful, who form one body in Christ (cf. 1 Cor 10:17), is both expressed and brought about”. A causal influence of the Eucharist is present at the Church's very origins. The Evangelists specify that it was the Twelve, the Apostles, who gathered with Jesus at the Last Supper (cf. Mt 26:20; Mk 14:17; Lk 22:14). This is a detail of notable importance, for the Apostles “were both the seeds of the new Israel and the beginning of the sacred hierarchy”. By offering them his body and his blood as food, Christ mysteriously involved them in the sacrifice which would be completed later on Calvary. By analogy with the Covenant of Mount Sinai, sealed by sacrifice and the sprinkling of blood, the actions and words of Jesus at the Last Supper laid the foundations of the new messianic community, the People of the New Covenant. The Apostles, by accepting in the Upper Room Jesus' invitation: “Take, eat”, “Drink of it, all of you” ( Mt 26:26-27), entered for the first time into sacramental communion with him. From that time forward, until the end of the age, the Church is built up through sacramental communion with the Son of God who was sacrificed for our sake: “Do this is remembrance of me... Do this, as often as you drink it, in remembrance of me” ( 1 Cor 11:24-25; cf. Lk 22:19).
Source: Ecclesia de Eucharistia (Vatican.va)