Quoting the Roman Catholic Catechism:
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 Roman Catholic Catechism ]