57 In accordance with the hope expressed by Pope Paul VI, our declared purpose is to re-establish together full unity in legitimate diversity: "God has granted us to receive in faith what the Apostles saw, understood, and proclaimed to us. By Baptism 'we are one in Christ Jesus ' ( Gal 3:28). In virtue of the apostolic succession, we are united more closely by the priesthood and the Eucharist. By participating in the gifts of God to his Church we are brought into communion with the Father through the Son in the Holy Spirit ... In each local Church this mystery of divine love is enacted, and surely this is the ground of the traditional and very beautiful expression 'Sister Churches', which local Churches were fond of applying to one another (cf. Decree, Unitatis Redintegratio , 14). For centuries we lived this life of 'Sister Churches', and together held Ecumenical Councils which guarded the deposit of faith against all corruption. And now, after a long period of division and mutual misunderstanding, the Lord is enabling us to discover ourselves as 'Sister Churches' once more, in spite of the obstacles which were once raised between us". If today, on the threshold of the third millennium, we are seeking the re-establishment of full communion, it is for the accomplishment of this reality that we must work and it is to this reality that we must refer. Contact with this glorious tradition is most fruitful for the Church. As the Council points out: "From their very origins the Churches of the East have had a treasury from which the Church of the West has amply drawn for its liturgy, spiritual tradition and jurisprudence". Part of this "treasury" are also "the riches of those spiritual traditions to which monasticism gives special expression. From the glorious days of the Holy Fathers, there flourished in the East that monastic spirituality which later flowed over into the Western world". As I have had the occasion to emphasize in my recent Apostolic Letter Orientale Lumen , the Churches of the East have lived with great generosity the commitment shown by monastic life, "starting with evangelization, the highest service that the Christian can offer his brother, followed by many other forms of spiritual and material service. Indeed it can be said that monasticism in antiquity—and at various times in subsequent ages too—has been the privileged means for the evangelization of peoples". The Council does not limit itself to emphasizing the elements of similarity between the Churches in the East and in the West. In accord with historical truth, it does not hesitate to say: "It is hardly surprising if sometimes one tradition has come nearer than the other to an apt appreciation of certain aspects of the revealed mystery or has expressed them in a clearer manner. As a result, these various theological formulations are often to be considered as complementary rather than conflicting". Communion is made fruitful by the exchange of gifts between the Churches insofar as they complement each other.
Source: Ut Unum Sint (Vatican.va)