Mystical Body of Christ (OSV Encyc. 1997)
The Church cannot be understood just by examining her visible elements. An institution, the Church is particularly a living organism, with spiritual and mystical dimensions which are much more important than any human analysis reveals.
"The Church is the Body of Christ. Through the Spirit and his action in the sacraments, above all the Eucharist, Christ, who once was dead and is now risen, establishes the community of believers as his own Body" (CCC 805).
This expression of the nature of the Church is not a mere metaphor. As persecutor of the early Christians, Saul of Tarsus thought he was opposing an institution, and discovered it was a Person: "I am Jesus, whom you are persecuting" (Acts 9:5). "I am Jesus...": Christ and those who make up his Church, are one. From this experience Paul developed his theology of the Body of Christ: Christ the Head of the Body, and we his members (cf. Eph 1:22-23; I Cor 12:8-11; 2 Cor 5:14-15).
Christ - God become Man - not only redeems us from sin, but incorporates us into himself, and so we become sons and daughters of God. We really share in his human-divine life as members of his Body. As in the human body, the vitality (which means union and strength) is shown in the different functions of its members, but especially in the organic connection between them, and above all in the links of each one with the Head (Col 1:18).
Each Christian then has a personal, voluntary, organic, vital, life-giving participation in this Body. So the members of the Church do not live unconnected lives. As in a true organism, the same life circulates among them, carrying strength to each according to his or her need. To separate oneself voluntarily from that life-giving circulation is to invite death.