54 We have now seen, from the brief historical relation of events in this letter, how many and how great were the benefits and blessings brought to the Ruthenian people by its union with the Catholic Church. This is not, indeed, to be wondered at; for if "it was God's good pleasure to let all completeness dwell" in Christ, no one can enjoy this completeness who is separated from the Church which "is his body." As our predecessor Pelagius II asserts, "whoever is not in the peace and unity of the Church will not be able to possess God." We have seen, too, that this beloved Ruthenian people has had to suffer great hardships, perils and vexations in defending to the best of its power its Catholic unity, but from these Divine Providence has freed it again and again and restored peace to it.
Source: Orientales Omnes Ecclesias (Vatican.va)