50 In this regard, it must first be acknowledged, with particular gratitude to Divine Providence, that our bonds with the Churches of the East, weakened in the course of the centuries, were strengthened through the Second Vatican Council. The observers from these Churches present at the Council, together with representatives of the Churches and Ecclesial Communities of the West, stated publicly, at that very solemn moment for the Catholic Church, their common willingness to seek the re-establishment of communion. The Council, for its part, considered the Churches of the East with objectivity and deep affection, stressing their ecclesial nature and the real bonds of communion linking them with the Catholic Church. The Decree on Ecumenism points out: "Through the celebration of the Eucharist of the Lord in each of these Churches, the Church of God is built up and grows in stature". It adds, as a consequence, that "although these Churches are separated from us, they possess true sacraments, above all — by apostolic succession — the priesthood and the Eucharist, whereby they are still joined to us in a very close relationship". Speaking of the Churches of the East, the Council acknowledged their great liturgical and spiritual tradition, the specific nature of their historical development, the disciplines coming from the earliest times and approved by the Holy Fathers and Ecumenical Councils, and their own particular way of expressing their teaching. The Council made this acknowledgement in the conviction that legitimate diversity is in no way opposed to the Church's unity, but rather enhances her splendour and contributes greatly to the fulfilment of her mission. The Second Vatican Ecumenical Council wished to base dialogue on the communion which already exists, and it draws attention to the noble reality of the Churches of the East: "Therefore, this Sacred Synod urges all, but especially those who plan to devote themselves to the work of restoring the full communion that is desired between the Eastern Churches and the Catholic Church, to give due consideration to these special aspects of the origin and growth of the Churches of the East, and to the character of the relations which obtained between them and the Roman See before the separation, and to form for themselves a correct evaluation of these facts".
Source: Ut Unum Sint (Vatican.va)