12 In order that this particular work may be joined intimately to the other duties of your pastoral office, see to it that the Missionary Union of the Clergy be established in your diocese, and if this has already been done, encourage the organization by your counsels, your exhortations, and your authority to renewed activity. This Union, founded providentially eight years ago by Our immediate Predecessor, has been enriched by numerous indulgences and put under the special jurisdiction of the Sacred Congregation of the Propaganda. In these last years it has spread through many dioceses of the Catholic world. We Ourselves have bestowed upon the Union, and this more than once, testimonials of Our pontifical well-wishes. Great is the number of priests, as well as ecclesiastical students, who belong to the Missionary Union and who pray, each according to his particular state in life, especially at the Holy sacrifice of the Mass, and encourage likewise others to pray, that the gift of faith be bestowed upon the almost limitless number of pagans. On every possible occasion they preach to the people about the apostolate which is carried on among the heathen. They also see to it that at certain specified times conferences, which are of great and lasting value, are held on mission work. They distribute mission literature and wherever they discover some one who gives indications of possessing a vocation to the missionary life, they assist him in obtaining the preparation necessary for such work. They encourage and promote, too, in every possible way, each within the limits of his own diocese, both the work of the Society for the Propagation of the Faith and its two allied and subsidiary activities.
Source: Rerum Ecclesiae (Vatican.va)