If we would define and describe this true Church of Jesus Christ – which is the One, Holy, Catholic, Apostolic and Roman Church – we shall find nothing more noble, more sublime, or more divine than the expression “the Mystical Body of Christ” – an expression which springs from and is, as it were, the fair flowering of the repeated teaching of the Sacred Scriptures and the Holy Fathers. . . .
But a body calls also for a multiplicity of members, which are linked together in such a way as to help one another. And as in the body when one member suffers, all the other members share its pain, and the healthy members come to the assistance of the ailing, so in the Church the individual members do not live for themselves alone, but also help their fellows, and all work in mutual collaboration for the common comfort and for the more perfect building up of the whole Body. . . .
One must not think, however, that this ordered or “organic” structure of the body of the Church contains only hierarchical elements and with them is complete; or, as an opposite opinion holds, that it is composed only of those who enjoy charismatic gifts – though members gifted with miraculous powers will never be lacking in the Church. That those who exercise sacred power in this Body are its chief members must be maintained uncompromisingly. It is through them, by commission of the Divine Redeemer Himself, that Christ’s apostolate as Teacher, King and Priest is to endure. At the same time, when the Fathers of the Church sing the praises of this Mystical Body of Christ, with its ministries, its variety of ranks, its officers, it conditions, its orders, its duties, they are thinking not only of those who have received Holy Orders, but of all those too, who, following the evangelical counsels, pass their lives either actively among men, or hidden in the silence of the cloister, or who aim at combining the active and contemplative life according to their Institute; as also of those who, though living in the world, consecrate themselves wholeheartedly to spiritual or corporal works of mercy, and of those in the state of holy matrimony. Indeed, let this be clearly understood, especially in our days, fathers and mothers of families, those who are godparents through Baptism, and in particular those members of the laity who collaborate with the ecclesiastical hierarchy in spreading the Kingdom of the Divine Redeemer occupy an honorable, if often a lowly, place in the Christian community, and even they under the impulse of God and with His help, can reach the heights of supreme holiness, which, Jesus Christ has promised, will never be wanting to the Church.
Now we see that the human body is given the proper means to provide for its own life, health and growth, and for that of all its members. Similarly, the Savior of mankind out of His infinite goodness has provided in a wonderful way for His Mystical Body, endowing it with the Sacraments, so that, as though by an uninterrupted series of graces, its members should be sustained from birth to death, and that generous provision might be made for the social needs of the Church. Through the waters of Baptism those who are born into this world dead in sin are not only born again and made members of the Church, but being stamped with a spiritual seal they become able and fit to receive the other Sacraments. – from Mystici Corporis Christi (1934)