How is the worldwide common union of the Orthodox Catholic Churches organized?

In Orthodox theology, the church is the localized body of Christians, constituted and manifested by the celebration of the Eucharist, which in turn requires the ministry of the bishop with his presbyters, deacons, and people. A church is therefore associated with a city, where the bishop’s teaching chair (cathedra, whence “cathedral”) is established: this city is called the bishop’s see. It is essential that there be only “one bishop in each catholic church”, “we are not ignorant that there is one God, one Christ the Lord whom we have confessed, one Holy Spirit, and that in the catholic church there ought to be one bishop” (Letter of Cornelius of Rome to Cyprian, in Eusebius, Ecclesiastical History, VI.43), or, as the Council of Nicaea decreed, “that there may not be two bishops in the city” (Nicaea I, canon 8; see Q070, Q071).

Because bishops are, by ordination and vocation, in communion with the neighboring cities, groups of Churches are formed according to practical principles of good order. Typically, the bishop of the chief city of a region (the metropolis) presides as first-among-equals among his fellow bishops, not least because the bishops assemble in his city, and this bishop (metropolitan or archbishop) is granted specific prerogatives by his fellow bishops, according to the principle of the thirty-fourth Apostolic Canon:

The bishops of every nation must know who is first (protos) among them, and recognize him as their head, and do nothing of consequence without his consent… but neither let him do anything without the consent of all: for so there will be concord, and God will be glorified through the Lord in the Holy Spirit (Apostolic Canon 34; see the Study: The Structure of the Orthodox Communion).

In some cases, these groups of Churches form still larger ecclesiastical bodies, some called patriarchates, whose presiding bishop is styled patriarch. Despite the different functional titles, all bishops still share equally in the one episcopate, “which is one, each part of which is held by each one for the whole” (St Cyprian, On the Unity of the Catholic Church, 5).

In application of this principle of organization, in which the church (diocese) is the basic unit, the structure of the autocephalous Churches today is as follows: the Ecumenical Patriarchate of Constantinople; the Patriarchates of Alexandria, Antioch, and Jerusalem; the Patriarchates of Moscow, Serbia, Romania, Bulgaria, and Georgia; the Churches of Cyprus and of Greece; the Churches of Poland, Albania, and the Czech Lands and Slovakia; and the Orthodox Church in America, together with the recent autocephalies discussed in the Study: The Structure of the Orthodox Communion, where the autonomous Churches are also listed. Regarding the use of ‘Church‘ for an archdiocese or metropolia, Metropolitan John (Zizioulas) of Pergamon clarifies:

A metropolis, an archdiocese, or a patriarchate cannot be called a Church in itself, but only by extension, by virtue of the fact that it is based on one or more episcopal dioceses, the local Churches, which alone, on account of the episcopal Eucharist, are properly called Churches (John Zizioulas, Being as Communion, pp. 252–253, n. 7)

(Q280a. How is Orthodox unity expressed?)

Orthodox Christians must always be attached to a parish, and therefore to a bishop, commemorated by name in the petitions, who must himself be in communion with the rest of the Orthodox episcopate.

By this means, Orthodox Christians in good standing may receive the Holy Mysteries in any canonical Orthodox parish anywhere in the world, provided they introduce themselves to the presiding presbyter and comply with local practice, a daily, practical experience of the one communion.

The unity of the Orthodox communion is most vividly expressed when the primate of an autocephalous Church celebrates the Divine Liturgy: at the Great Entrance, he commemorates by name every other primate of the holy Churches of God, according to the order of the diptychs, the audible sign that the many Churches are one common union, sharing the one Bread.