The answers in this section on autocephaly were provided by a seminary faculty member in a 1970 OCA publication.
Are autocephalous churches always churches of separate nations?
One can see from the list of the contemporary autocephalous churches that not all of them are national churches. The Patriarchate of Antioch, for example, covers several nations; while the Church of Georgia exists politically within the same state as the Church of Russia.
It can be said generally, however, that in recent times the new autocephalous churches emerged within the territories of the new nations, having boundaries coextensive with those of the nation itself. That is not true of the Orthodox Church in America, of course, which covers more than one country.
In the time of the Roman-Byzantine Empire which was, one might say, “one nation” as far as civil government was concerned, various territories within the empire had their own self-governing churches either because of the political divisions of the empire, or because of “ancient customs,” or because of some special privileges or needs.
In the Early Church, we should notice, that for all practical purposes the church of each city was autocephalous. Of course the word was not used (it is of relatively recent usage), but the documents of the time, including the New Testament itself, show that each church community, headed by its bishop with its priests (presbyters), deacons and faithful people, governed itself and understood itself to be fully the “Catholic Church,” i.e, whole and complete with nothing essential lacking from its fulness of faith and sacramental life.
The unity of the first Christian churches in the various cities was not accomplished by legal authority or juridical power. It was accomplished by the simple fact that each church had exactly the same faith and life sealed by the same Christian mysteries (sacraments). This identity of the Church in space and time was assured by the consecration of the new bishop of each church at the divine liturgy by as many bishops of other churches as possible.
Only as the number of churches increased throughout the empire and became officially connected to the life of the state did such things as metropolitan districts and patriarchates grow up. And then, finally, we have the national churches of modern times after the end of the imperial era.