3. The laity derive the right and duty to the apostolate from their union with Christ the head; incorporated into Christ's Mystical Body through Baptism and strengthened by the power of the Holy Spirit through Confirmation, they are assigned to the apostolate by the Lord Himself. They are consecrated for the royal priesthood and the holy people (cf. 1 Peter 2:4-10) not only that they may offer spiritual sacrifices in everything they do but also that they may witness to Christ throughout the world. The sacraments, however, especially the most holy Eucharist, communicate and nourish that charity which is the soul of the entire apostolate.
One engages in the apostolate through the faith, hope, and charity which the Holy Spirit diffuses in the hearts of all members of the Church. Indeed, by the precept of charity, which is the Lord's greatest commandment, all the faithful are impelled to promote the glory of God through the coming of His kingdom and to obtain eternal life for all men--that they may know the only true God and Him whom He sent, Jesus Christ (cf. John 17:3). On all Christians therefore is laid the preeminent responsibility of working to make the divine message of salvation known and accepted by all men throughout the world.
For the exercise of this apostolate, the Holy Spirit Who sanctifies the people of God through ministry and the sacraments gives the faithful special gifts also (cf. 1 Cor. 12:7), "allotting them to everyone according as He wills" (1 Cor. 12:11) in order that individuals, administering grace to others just as they have received it, may also be "good stewards of the manifold grace of God" (1 Peter 4:10), to build up the whole body in charity (cf. Eph. 4:16). From the acceptance of these charisms, including those which are more elementary, there arise for each believer the right and duty to use them in the Church and in the world for the good of men and the building up of the Church, in the freedom of the Holy Spirit who "breathes where He wills" (John 3:8). This should be done by the laity in communion with their brothers in Christ, especially with their pastors who must make a judgment about the true nature and proper use of these gifts not to extinguish the Spirit but to test all things and hold for what is good (cf. 1 Thess. 5:12, 19, 21).
Source: Decree on the Apostolate of the Laity, Apostolicam Actuositatem.