2 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. In the Church there is a diversity of ministry but a oneness of mission. Christ conferred on the Apostles and their successors the duty of teaching, sanctifying, and ruling in His name and power. But the laity likewise share in the priestly, prophetic, and royal office of Christ and therefore have their own share in the mission of the whole people of God in the Church and in the world. They exercise the apostolate in fact by their activity directed to the evangelization and sanctification of men and to the penetrating and perfecting of the temporal order through the spirit of the Gospel. In this way, their temporal activity openly bears witness to Christ and promotes the salvation of men. Since the laity, in accordance with their state of life, live in the midst of the world and its concerns, they are called by God to exercise their apostolate in the world like leaven, with the ardor of the spirit of Christ.
Source: Apostolicam Actuositatem (Vatican.va)