17 As for the defense of religion, if you wish to undertake it in a worthy manner, and to carry it on perseveringly and efficaciously, two things are first of all necessary: you must model yourselves so faithfully on the precepts of the Christian law that all your actions and your entire lives may do honor to the faith you profess, and then you must be closely united with those whose special office it is to watch over religion, with your priests, your bishops, and above all with this Apostolic See, which is the pivot of the Catholic faith and of all that can be done in its name. Thus armed for the fray, go forth fearlessly for the defense of the Church; but take care that your trust is placed entirely in God, for whose cause you are working, and never cease to pray to Him for help.
Source: Vehementer Nos (Vatican.va)