2 It is a well-known fact that Our Predecessors fulfilled at all times the Divine Commission wherewith they were charged of teaching and baptizing all nations, that the priests sent by them (many of whom the Church publicly venerates because of the holiness of their lives or because they so courageously suffered martyrdom) zealously strove with varying results to enlighten by the Faith first Europe, and, later on, even unknown lands, and this almost immediately after their discovery. We say "with varying results," for it sometimes happened that the missionaries after laboring with little or no effect were either put to death or driven out of the country. As a result, the field which they had begun to cultivate, at times a mere wilderness but at other times already converted by them into a veritable garden of roses, when left to itself was once again overrun with thorns and briars. In spite of all this, it is a great consolation to see how in recent years the Congregations which are devoted to foreign missionary work have actually redoubled their labors and have gained such memorable fruits from their work, and how the faithful, on their part, have so generously responded to this increased missionary effort with a great increase in the amount of alms given for such holy purposes. There is no doubt that this renewed activity was greatly promoted by the Apostolic Letter of November 30, 1919, entitled, On the Propagation of the Catholic Faith Throughout the World , which Our Predecessor of happy memory directed to the episcopate of the whole world. In this letter, while the Pontiff on the one hand stimulated the diligence and zeal of all the bishops in the work of obtaining help for the missions, he did not fail to point out, and very wisely, to Apostolic Vicars and Prefects, the obstacles to be avoided and the methods to be followed by their clergy in order to render more fruitful the exercise of the sacred apostolate.
Source: Rerum Ecclesiae (Vatican.va)