48 From the beginning holy Church by her very nature has been compelled to spread the Word of God everywhere, and in fulfilling this obligation to which she knows not how to be unfaithful she has never ceased to ask for a threefold assistance from her children: namely, prayers, material aid, and, in some cases, the gift of themselves. At the present day, too, her missionary activities, especially in Africa, demand this threefold assistance from the Catholic world.
Source: Fidei Donum (Vatican.va)