27 The quasi-common heritage of the Gospel and the common duty of Christian witness resulting from it recommend and frequently require the cooperation of Catholics with other Christians, on the part of individuals and communities within the Church, either in activities or in associations, in the national or international field. Likewise, common human values not infrequently call for cooperation between Christians pursuing apostolic aims and those who do not profess Christ's name but acknowledge these values. By this dynamic and prudent cooperation, which is of special importance in temporal activities, the laity bear witness to Christ, the Savior of the world, as well as to the unity of the human family.
Source: Apostolicam Actuositatem (Vatican.va)