From the beginning, Jesus associated his disciples with his own life, revealed the mystery of the Kingdom to them, and gave them a share in his mission, joy, and sufferings. Jesus spoke of a still more intimate communion between him and those who would follow him: “Abide in me, and I in you…. I am the vine, you are the branches.” and he proclaimed a mysterious and real communion between his own body and ours: “He who eats my flesh and drinks my blood abides in me, and I in him.”
When his visible presence was taken from them, Jesus did not leave his disciples orphans. He promised to remain with them until the end of time; he sent them his Spirit. As a result communion with Jesus has become, in a way, more intense: “By communicating his Spirit, Christ mystically constitutes as his body those brothers of his who are called together from every nation.”
The comparison of the Church with the body casts light on the intimate bond between Christ and his Church. Not only is she gathered around him; she is united in him, in his body. Three aspects of the Church as the Body of Christ are to be more specifically noted: the unity of all her members with each other as a result of their union with Christ; Christ as head of the Body; and the Church as bride of Christ.
Believers who respond to God’s word and become members of Christ’s Body, become intimately united with him: “In that body the life of Christ is communicated to those who believe, and who, through the sacraments, are united in a hidden and real way to Christ in his Passion and glorification.”This is especially true of Baptism, which unites us to Christ’s death and Resurrection, and the Eucharist, by which “really sharing in the body of the Lord, . . . we are taken up into communion with him and with one another.”
The body’s unity does not do away with the diversity of its members: “In the building up of Christ’s Body there is engaged a diversity of members and functions. There is only one Spirit who, according to his own richness and the needs of the ministries, gives his different gifts for the welfare of the Church.” The unity of the Mystical Body produces and stimulates charity among the faithful: “From this it follows that if one member suffers anything, all the members suffer with him, and if one member is honored, all the members together rejoice.” Finally, the unity of the Mystical Body triumphs over all human divisions: “For as many of you as were baptized into Christ have put on Christ. There is neither Jew nor Greek, there is neither slave nor free, there is neither male nor female; for you are all one in Christ Jesus.”
From the Catechism of the Catholic Church 787-791