54 The other event which I am pleased to recall is the celebration of the Millennium of the Baptism of Rus' (988-1988). The Catholic Church, and this Apostolic See in particular, desired to take part in the Jubilee celebrations and also sought to emphasize that the Baptism conferred on Saint Vladimir in Kiev was a key event in the evangelization of the world. The great Slav nations of Eastern Europe owe their faith to this event, as do the peoples living beyond the Ural Mountains and as far as Alaska. In this perspective an expression which I have frequently employed finds its deepest meaning: the Church must breathe with her two lungs! In the first millennium of the history of Christianity, this expression refers primarily to the relationship between Byzantium and Rome. From the time of the Baptism of Rus' it comes to have an even wider application: evangelization spread to a much vaster area, so that it now includes the entire Church. If we then consider that the salvific event which took place on the banks of the Dnieper goes back to a time when the Church in the East and the Church in the West were not divided, we understand clearly that the vision of the full communion to be sought is that of unity in legitimate diversity. This is what I strongly asserted in my Encyclical Epistle Slavorum Apostoli on Saints Cyril and Methodius and in my Apostolic Letter Euntes in Mundum addressed to the faithful of the Catholic Church in commemoration of the Millennium of the Baptism of Kievan Rus'.
Source: Ut Unum Sint (Vatican.va)