66 It is not enough, however, to make your people more zealous in supporting this work; much more is required. Not a few dioceses are so well supplied with clergy, thanks be to God, that no loss would be felt if some of their priests should enter the mission field. To such dioceses We would apply, with fatherly solicitude, the Gospel saying: what you have that is over and above your needs, give that to the poor. Our heart goes out to Our brother bishops who are filled with distress and fear as they see the number of candidates for the priesthood and the religious life growing fewer, and are thus unable to provide properly for the spiritual needs of their own flocks. We share their anxiety and say to them as St. Paul did to the Corinthians: "Not that the relief of others should become your burden, but that there should be equality."
Source: Fidei Donum (Vatican.va)