21 Indeed also God himself expects something great and of the first rank from his ministers. It is that they diligently prepare the fields and cultivate them, so that such little trees can sprout. It is the task of priests to propagate among the faithful a knowledge of the mission apostolate; as Our predecessor Pius XI admonished, no one should be considered a priest w ho does not burn with charity for the missions.
Source: Saeculo Exeunte Octavo (Vatican.va)