65 You no doubt remember how warmly the encyclical Letter Rerum Ecclesiae recommended the Missionary Union of the Clergy, whose object is to unite the combined energy of clergy, secular and regular, and of ecclesiastical students in furthering the cause of the missions in every possible way. Having had the happiness to witness the success of this Union, as We have mentioned, We earnestly desire that it increase and spread ever more widely and arouse both priests and people to work ever more zealously for the cause of the missions. This Union is the source from which depends the succcess of the other Pontifical Societies of the Propagation of the Faith, of St. Peter Apostle for Native Clergy, and of the Holy Childhood. There is no need for Us at present to dwell on the importance, necessity and outstanding merits of these societies, which Our Predecessors have enriched with numerous indulgences. We fully approve that the faithful be asked to contribute generously, especially on Mission Sunday. But We desire first and foremost that all pray to Almighty God, that they help those called to missionary work, and that they join and promote as much as possible the Pontifical Societies We have mentioned. You are quite aware, Venerable Brethren, that We recently instituted a special children's festival to help the Society of the Holy Childhood with prayers and alms. These little children of Ours are thus accustomed to pray earnestly for the salvation of the infidel; and may it be the means of sowing the seed of a missionary vocation in their innocent hearts and of fostering its growth.
Source: Evangelii Praecones (Vatican.va)