The Catholic Church established by Christ as an extension and continuation of the Incarnation.
In the words of Pius XII, "If we would define and describe the true Church of Jesus Christ -- which is the one, holy, Catholic, apostolic Roman Church -- we shall find nothing more noble, more sublime, or more divine than the expression 'the Mystical Body of Jesus Christ' -- an expression that flows spontaneously from the repeated teaching of the sacred Scriptures and the holy Fathers."
The term "body," when referring to the Church, derives its meaning from the analogy used by St. Paul, where he speaks of Christians: "You are the Body of Christ, member for member" (I Corinthians 12:27), and of Christ: "the Head of His Body, the Church" (Colossians 1:18).
Corollary to being a body, the Church must have a multiplicity of discernible members because the possession of parts is an essential feature of anything bodily. And just as a natural body is formed of different organs with different functions arranged in due order, so the Church is bound together by the combination of structurally united parts, and has a variety of members that are reciprocally dependent. Another name for this interdependence is the hierarchy, with its graded levels of orders and jurisdiction, of superiors and subjects, beginning with the Sovereign Pontiff and terminating in the laity.
The body (soma) that St. Paul identifies with the Church is a living reality, and like every organism requires suitable means to enter into life, to grow and mature and prosper according to its nature. Similarly in the Catholic Church, the sacraments are available for every spiritual need and circumstance of human life.
Moreover, the Roman Catholic Church regards herself as the Body of Christ. He was the originator of the Church by his preaching and choice of the Apostles to carry on his work, by his death on the Cross when he merited the graces to be channeled through the Mystical Body, and by the descent of the Holy Spirit, whom he sent on Pentecost. He continues to rule the Church from within by supernatural means that are permanent and constantly active within the members.
The Church is called Mystical because she is a mystery, which God revealed to be true but whose inner essence must be accepted on faith and without full comprehension by the mind. Otherwise than in other societies, the end or purpose of the church is not temporal or earthly but heavenly and eternal; its spiritual bond is the will of God; incorporation in the Church effects a profound internal change in the members; and the whole reality is called supernatural because it leads to the destiny of seeing God in the beatific vision after death. But the Roman Catholic Church is mainly said to be the Mystical Body of Christ because it is sacramental. The Church is the great sacrament of the New Law, instituted by Christ for the communication of invisible grace to the whole world.