It is true that today there are people who speak of the union of the Churches. But this term is worthless theologically. We cannot speak of union, but of a unity of faith. We cannot speak of Churches which are separated and struggling to reach the truth and union, but about the Church which is always united with Christ and has never lost the truth, and about people who have broken away from it.
Some people who speak of union of the Churches use to satiety Christ's archpriestly prayer, which is in the Gospel according to John, and especially the point where Christ asks the Father that the disciples "may be one" and "that they all may be one" (John 17, 20-22). But if anyone reads the whole text attentively, he will discover that Christ is not referring to a union of the Churches which will come about in the future, but to the union of the Disciples which will come about on the day of Pentecost, when they will receive the Holy Spirit. This text speaks of the glorification of the Apostles which took place at Pentecost. Actually, at Pentecost the Apostles became members of the Body of Christ, they saw the glory of God, they reached deification, and so attained unity together in the single Body of Christ. Anyone who experiences Pentecost in his personal life attains this unity. The Apostle Paul, although he was not present with the Apostles on the day of Pentecost, is portrayed by the Church in the icon of Pentecost, because he too reached the vision of Christ and therefore has unity with the other Apostles.
—Metr. Hierotheos of Nafpaktos, The Mind of the Orthodox Church, Chapter II
See: Harsh Truth