28 The apostolate can attain its maximum effectiveness only through a diversified and thorough formation. This is demanded not only by the continuous spiritual and doctrinal progress of the lay person himself but also by the accommodation of his activity to circumstances varying according to the affairs, persons, and duties involved. This formation for the apostolate should rest upon those bases which have been stated and proclaimed by this most holy council in other documents. In addition to the formation which is common for all Christians, many forms of the apostolate demand also a specific and particular formation because of the variety of persons and circumstances.
Source: Apostolicam Actuositatem (Vatican.va)