5 This sort of consideration, a very serious one indeed, has been advanced more than once by Our Predecessors, and We Ourselves, as you well know, have been most earnest in touching upon it. It should inspire all Catholics with apostolic zeal, as their awareness of having received the faith demands. Let them direct this zeal toward those regions of Europe in which the Christian religion has been cast off, or to the boundless spaces of South America; in both of these continents there are great difficulties to be overcome, as We know well. Let them give financial assistance to the Catholics of Oceania and to the missions in Asia; such assistance is of the utmost importance, especially in those countries where the battles of the Lord are being fought so fiercely. Let them likewise perform the duties of fraternal charity in behalf of those countless Christians who are very dear to Us and are the glory of the Church, since they have earned the evangelical beatitude proper to those "who suffer persecution for justice' sake." Let them grieve for the lamentable state of innumerable souls, especially of those young people who because of the atheistic propaganda of our times are growing up in the wretched condition of complete ignorance of religion and, in some cases, of active hatred of God.
Source: Fidei Donum (Vatican.va)