This article attempts an approach to discuss the mystery and the unity of the church and firstly, it underlined that the church cannot have a formal definition as the divine life extended from Christ's resurrected body into those who believe and receive the Holy Mysteria. At the same time, the process of becoming part of the church is a mystical one. In order for life in Christ to be possible, Christ must be formed in the human being. Becoming a Christian (...) is described in terms specific to the process of birth, its end being the formation of Christ in us. Then, the theme of unity is dealt with from an Eastern Orthodox perspective, not in isolation, but in connection with the Roman Catholic, Protestant and ecumenical concerns for unity and collaboration amongst Christians. The cosmic unifying vocation of the church was accomplished because of the five syntheses realised by her Head, our Lord and Saviour Jesus Christ. Then there are references to the Theotokological aspect of the church, Holy Baptism as the bond of church unity; Holy Eucharist as the Mysterion of ecclesial unity, the ministry of the bishop for unity and the unity of Christians in the one Church. CONTRIBUTION: The author, having experience in ecumenical dialogues as a former member of the Commmission on World Mission and Evangelism and, at present, of the Commission on Faith and Order of the World Council of Churches, tried to offer a balanced perspective on the sensitive theme of the unity of the Christians in the one Church, referring to some of the main relevant sources, which facilitate dialogue, mutual knowledge and collaboration. (shrink)