5 We think it will be useful if in this letter we give a brief historical summary of the events with which we are concerned. It must be observed in the first place that even before the union of the Ruthenians with the Apostolic See was happily concluded at Rome and confirmed at Brest in 1595 and 1596, these people had more than once looked to the Roman Church as the one mother of the whole Christian community and dutifully paid it due obedience and reverence. Thus, for instance, the noble prince who is reverenced by the numberless peoples of the Russian empire as the author and promoter of their conversion to the Christian faith, St. Vladimir, borrowed liturgical rites and sacred ceremonies from the Eastern Church; but he not only dutifully persevered in the unity of the Catholic Church, but also took pains to establish friendly relations between his nation and the Apostolic See. Not a few of his descendants also received the legates of the Roman pontiffs with due honor and were fraternally united with other Catholic communities, even after the Church of Constantinople was separated by the lamentable schism.
Source: Orientales Omnes Ecclesias (Vatican.va)