24 In the Apostolic Letter Maximum illud of Our Predecessor of immortal memory, Benedict XV, given in the year 1919, and in the Encyclical Letter Rerum Ecclesiae , of Our immediate Predecessor of happy memory, Pius XI, it was laid down that the missions would have as the final goal of their activities the establishment of the Church in new territiories. And We Ourselves when, as We have said, received in audience the directors of mission activities in 1944, made the following statement: "The magnanimous and noble purpose which missionaries have is the propagation of the faith in new lands in such a way that the Church may ever become more firmly established in them and as soon as possible reach such a stage of development that it can continue to exist and flourish without the aid of missionary organizations. These missionary organizations do not serve their own ends, but it is their task to use every endeavor to realize the lofty purpose We have already mentioned. When that has been attained, then let them be happy to turn to other fields." "Wherefore let the missionary take up no permanent abode in those places where the work of the apostolate has reached full development, since it is up to him to evangelize and sanctify the whole world. The missionary's appointed task is to promote ever more rapidly in district after district till the last man in the most remote corner of the earth has been reached, the Kingdom of the Divine Redeemer Who rose triumphant from the dead and to Whom is given all power in heaven and on earth.",
Source: Evangelii Praecones (Vatican.va)