73 Such is the positive task, embracing at once theory and practice, which the Church undertakes in virtue of the mission, confided to her by Christ, of constructing a Christian society, and, in our own times, of resisting unto victory the attacks of Communism. It is the duty of the Christian State to concur actively in this spiritual enterprise of the Church, aiding her with the means at its command, which although they be external devices, have nonetheless for their prime object the good of souls.
Source: Divini Redemptoris (Vatican.va)