13 You are assuredly aware, Venerable Brothers, of the large amounts of money which the Missionary Union of the Clergy has already collected to help these good works, and what wonderful prospects they have in the future of collecting even more, due to the increased generosity of the faithful from year to year. Some of you have been patrons and sponsors of the Missionary Union in your dioceses. It is Our supreme desire that from now on it will be impossible to point to a cleric who is not literally burning with love for the missions.
Source: Rerum Ecclesiae (Vatican.va)