55 In its historical survey the Council Decree Unitatis Redintegratio has in mind the unity which, in spite of everything, was experienced in the first millennium and in a certain sense now serves as a kind of model. "This most sacred Synod gladly reminds all ... that in the East there flourish many particular or local Churches; among them the Patriarchal Churches hold first place; and of these, many glory in taking their origin from the Apostles themselves". The Church's journey began in Jerusalem on the day of Pentecost and its original expansion in the oikoumene of that time was centred around Peter and the Eleven (cf. Acts 2:14). The structures of the Church in the East and in the West evolved in reference to that Apostolic heritage. Her unity during the first millennium was maintained within those same structures through the Bishops, Successors of the Apostles, in communion with the Bishop of Rome. If today at the end of the second millennium we are seeking to restore full communion, it is to that unity, thus structured, which we must look. The Decree on Ecumenism highlights a further distinctive aspect, thanks to which all the particular Churches remained in unity: "an eager desire to perpetuate in a communion of faith and charity those family ties which ought to thrive between local Churches, as between sisters".
Source: Ut Unum Sint (Vatican.va)