Christ “is the head of the body, the Church.” He is the principle of creation and redemption. Raised to the Father’s glory, “in everything he (is) preeminent,” especially in the Church, through whom he extends his reign over all things.
Christ unites us with his Passover: all his members must strive to resemble him, “until Christ be formed” in them. “For this reason we . . . are taken up into the mysteries of his life, . . . associated with his sufferings as the body with its head, suffering with him, that with him we may be glorified.”
Christ provides for our growth: to make us grow toward him, our head, he provides in his Body, the Church, the gifts and assistance by which we help one another along the way of salvation.
Christ and his Church thus together make up the “whole Christ” (Christus totus). the Church is one with Christ. the saints are acutely aware of this unity:
Let us rejoice then and give thanks that we have become not only Christians, but Christ himself. Do you understand and grasp, brethren, God’s grace toward us? Marvel and rejoice: we have become Christ. For if he is the head, we are the members; he and we together are the whole man…. the fullness of Christ then is the head and the members. But what does “head and members” mean? Christ and the Church.
Our redeemer has shown himself to be one person with the holy Church whom he has taken to himself.
Head and members form as it were one and the same mystical person.
A reply of St. Joan of Arc to her judges sums up the faith of the holy doctors and the good sense of the believer: “About Jesus Christ and the Church, I simply know they’re just one thing, and we shouldn’t complicate the matter.”
From the Catechism of the Catholic Church 792-795