THE VOCATION OF THE LAITY TO 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,(1) 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.
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).
Since Christ, sent by the Father, is the source and origin of the whole apostolate of the Church, the success of the lay apostolate depends upon the laity's living union with Christ, in keeping with the Lord's words, "He who abides in me, and I in him, bears much fruit, for without me you can do nothing" (John 15:5). This life of intimate union with Christ in the Church is nourished by spiritual aids which are common to all the faithful, especially active participation in the sacred liturgy.(5) These are to be used by the laity in such a way that while correctly fulfilling their secular duties in the ordinary conditions of life, they do not separate union with Christ from their life but rather performing their work according to God's will they grow in that union. In this way the laity must make progress in holiness in a happy and ready spirit, trying prudently and patiently to overcome difficulties.(6) Neither family concerns nor other secular affairs should be irrelevant to their spiritual life, in keeping with the words of the Apostle, "What-ever you do in word or work, do all in the name of the Lord Jesus Christ, giving thanks to God the Father through Him" (Col. 3:17).
Such a life requires a continual exercise of faith, hope, and charity. Only by the light of faith and by meditation on the word of God can one always and everywhere recognize God in Whom "we live, and move, and have our being" ( Acts 17:28), seek His will in every event, see Christ in everyone whether he be a relative or a stranger, and make correct judgments about the true meaning and value of temporal things both in themselves and in their relation to man's final goal.
They who have this faith live in the hope of the revelation of the sons of God and keep in mind the cross and resurrection of the Lord. In the pilgrimage of this life, hidden with Christ in God and free from enslavement to wealth, they aspire to those riches which remain forever and generously dedicate themselves wholly to the advancement of the kingdom of God and to the reform and improvement of the temporal order in a Christian spirit. Among the trials of this life they find strength in hope, convinced that "the sufferings of the present time are not worthy to be compared with the glory to come that will be revealed in us" (Rom. 8:18).
Impelled by divine charity, they do good to all men, especially to those of the household of the faith (cf. Gal. 6:10), laying aside "all malice and all deceit and pretense, and envy, and all slander" (1 Peter 2:1), and thereby they draw men to Christ. This charity of God, "which is poured forth in our hearts by the Holy Spirit who has been given to us" (Rom. 5:5), enables the laity really to express the spirit of the beatitudes in their lives. Following Jesus in His poverty, they are neither depressed by the lack of temporal goods nor inflated by their abundance; imitating Christ in His humility, they have no obsession for empty honors (cf. Gal. 5:26) but seek to please God rather than men, ever ready to leave all things for Christ's sake (cf. Luke 14:26) and to suffer persecution for justice sake (cf. Matt. 5:10), as they remember the words of the Lord, "If anyone wishes to come after me, let him deny himself and take up his cross and follow me" (Matt. 16:24) . Promoting Christian friendship among themselves, they help one another in every need whatsoever.
This plan for the spiritual life of the laity should take its particular character from their married or family state or their single or widowed state, from their state of health, and from their professional and social activity. They should not cease to develop earnestly the qualities and talents bestowed on them in accord with these conditions of life, and they should make use of the gifts which they have received from the Holy Spirit.
Furthermore, the laity who have followed their vocation and have become members of one of the associations or institutes approved by the Church try faithfully to adopt the special characteristics of the spiritual life which are proper to them as well. They should also hold in high esteem professional skill, family and civic spirit, and the virtues relating to social customs, namely, honesty, justice, sincerity, kindness, and courage, without which no true Christian life can exist.
The perfect example of this type of spiritual and apostolic life is the most Blessed Virgin Mary, Queen of Apostles, who while leading the life common to all here on earth, one filled with family concerns and labors, was always intimately united with her Son and in an entirely unique way cooperated in the work of the Savior. Having now been assumed into heaven, with her maternal charity she cares for these brothers of her Son who are still on their earthly pilgrimage and remain involved in dangers and difficulties until they are led into the happy fatherland. All should devoutly venerate her and commend their life and apostolate to her maternal care.
Culled from - http://www.vatican.va/archive/hist_councils/ii_vatican_council/documents/vat-ii_decree_19651118_apostolicam-actuositatem_en.html