1 To intensify the apostolic activity of the people of God, the most holy synod earnestly addresses itself to the laity, whose proper and indispensable role in the mission of the Church has already been dealt with in other documents. The apostolate of the laity derives from their Christian vocation and the Church can never be without it. Sacred Scripture clearly shows how spontaneous and fruitful such activity was at the very beginning of the Church (cf. Acts 11:19-21; 18:26; Rom. 16:1-16; Phil. 4:3). Our own times require of the laity no less zeal: in fact, modern conditions demand that their apostolate be broadened and intensified. With a constantly increasing population, continual progress in science and technology, and closer interpersonal relationships, the areas for the lay apostolate have been immensely widened particularly in fields that have been for the most part open to the laity alone. These factors have also occasioned new problems which demand their expert attention and study. This apostolate becomes more imperative in view of the fact that many areas of human life have become increasingly autonomous. This is as it should be, but it sometimes involves a degree of departure from the ethical and religious order and a serious danger to Christian life. Besides, in many places where priests are very few or, in some instances, deprived of due freedom for priestly work, the Church could scarcely exist and function without the activity of the laity. An indication of this manifold and pressing need is the unmistakable work being done today by the Holy Spirit in making the laity ever more conscious of their own responsibility and encouraging them to serve Christ and the Church in all circumstances. In this decree the Council seeks to describe the nature, character, and diversity of the lay apostolate, to state its basic principles, and to give pastoral directives for its more effective exercise. All these should be regarded as norms when the canon law, as it pertains to the lay apostolate, is revised.
Source: Apostolicam Actuositatem (Vatican.va)