"I am the vine, you are the branches" (Jn 15: 5). From the communion that Christians experience in Christ there immediately flows the communion which they experience with one another: all are branches of a single vine, namely, Christ. In this communion is the wonderful reflection and participation in the mystery of the intimate life of love in God as Trinity, Father, Son and Holy Spirit as revealed by the Lord Jesus. For this communion Jesus prays: "that they may all be one; even as you, Father, are in me, and I in you, that they also may be in us, so that the world may believe that you have sent me" (Jn 17: 21).
Such communion is the very mystery of the Church, as the Second Vatican Council recalls in the celebrated words of Saint Cyprian: "The Church shines forth as 'a people made one with the unity of the Father, Son and Holy Spirit'". We are accustomed to recall this mystery of Church communion at the beginning of the celebration of the Eucharist, when the priest welcomes all with the greeting of the Apostle Paul: "The grace of the Lord Jesus, the love of God and the fellowship of the Holy Spirit be with you all" (2 Cor 13:13).
St John Paul II, CHRISTIFIDELES LAICI, 18.