All efforts directed toward the mission of the Church are referred to as the apostolate, the role of being an apostle. All Christians, lay and ordained, participate in the apostolate.
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).
Decree on the Apostolate of the Laity, 2
Second Vatican Council