The Church was founded for the purpose of spreading the kingdom of Christ throughout the earth for the glory of God the Father, to enable all men to share in His saving redemption, and that through them the whole world might enter into a relationship with Christ. All activity of the Mystical Body directed to the attainment of this goal is called the apostolate, which the Church carries on in various ways through all her members. For the Christian vocation by its very nature is also a vocation to the apostolate. No part of the structure of a living body is merely passive but has a share in the functions as well as life of the body: so, too, in the body of Christ, which is the Church, "the whole body . . . in keeping with the proper activity of each part, derives its increase from its own internal development" (Eph. 4:16).
Indeed, the organic union in this body and the structure of the members are so compact that the member who fails to make his proper contribution to the development of the Church must be said to be useful neither to the Church nor to himself (Vatican II, Apostalicam Actuositatem, 2).