12 However, Christian communities to which missionaries still devote their zeal, although already governed by their own hierarchy, are still in need of the work of missionaries from other countries, either because of the vastness of the territory, or the increasing number of converts, or the multitude of those who have not yet benefited from the doctrine of the Gospel. To such missionaries, no doubt, apply these words of Our immediate predecessor: "These cannot be considered foreigners, for all Catholic priests who truly answer their vocation feel themselves native sons wherever they work, in order that the Kingdom of God may flourish and develop." (20) Let them therefore work united by the bond of that loving, brotherly, and sincere charity which mirrors the love they must feel toward the Divine Redeemer and His Church; and, in prompt and filial obedience to their Bishops, whom "the Holy Spirit placed . . . to rule the Church of God," (21) they must be "of one heart and one soul," (22) grateful to each other for the mutual cooperation and help; indeed, if they act in this manner, it should be apparent to everyone's eyes that they are the disciples of Him Who, in His own and most distinctive "new" commandments, exhorted all to a mutual and always increasing love. (23)
Source: Princeps Pastorum (Vatican.va)