13 The apostolate in the social milieu, that is, the effort to infuse a Christian spirit into the mentality, customs, laws, and structures of the community in which one lives, is so much the duty and responsibility of the laity that it can never be performed properly by others. In this area the laity can exercise the apostolate of like toward like. It is here that they complement the testimony of life with the testimony of the word. It is here where they work or practice their profession or study or reside or spend their leisure time or have their companionship that they are more capable of helping their brethren. The laity fulfill this mission of the Church in the world especially by conforming their lives to their faith so that they become the light of the world as well as by practicing honesty in all their dealings so that they attract all to the love of the true and the good and finally to the Church and to Christ. They fulfill their mission also by fraternal charity which presses them to share in the living conditions, labors, sorrows, and aspirations of their brethren with the result that the hearts of all about them are quietly prepared for the workings of saving grace. Another requisite for the accomplishment of their task is a full consciousness of their role in building up society whereby they strive to perform their domestic, social, and professional duties with such Christian generosity that their manner of acting should gradually penetrate the whole world of life and labor. This apostolate should reach out to all wherever they may be encountered; it should not exclude any spiritual or temporal benefit which they have the ability to confer. True apostles however, are not content with this activity alone but endeavor to announce Christ to their neighbors by means of the spoken word as well. For there are many persons who can hear the Gospel and recognize Christ only through the laity who live near them. Children also have their own apostolic work to do. According to their ability they are true living witnesses of Christ among their companions.
Source: Apostolicam Actuositatem (Vatican.va)