The national or ethnic divisions are due to historical factors in the United States. If you were in Greece, Russia, Bulgaria or some other traditional Eastern Orthodox area all every Church would be simply called an Orthodox Church. The Eastern Orthodox Churches in the United States uses these ethnic or geographical titles because our parishes were founded by immigrants from different countries with different cultures. Because we have always prayed in the language of the people, when the first immigrants arrived here they had to have a Priest of their nationality, or one who spoke their language. My parish was founded by immigrants from Lebanon and originally did the services in Arabic. Now no one in the parish speaks Arabic so we now use English and have many people who are not of Arabic ancestry including people who converted to Orthodoxy as I did.
Despite our administrative and ethnic divisions we consider ourselves one Church and work together. We all share the same doctrine and are in full Communion with each other. With the exception of a few Western Rite parishes under Antioch and the Moscow Patriarchate, we all use the Byzantine Rite. I received my theological education at a Greek Orthodox seminary although I am in the Antiochian Archdiocese. I was taught the same doctrine that I would have been taught had I gone to St. Vladimir’s which is under the Orthodox Church in America which is Russian Orthodox in backgroud.
Because each national Orthodox Church is self-governing different practices have developed. For example, tomorrow we begin the Christmas Fast. In the Greek Orthodox Archdiocese, they allow weddings until December 12. In the Antiochian Archdiocese in which I serve, we do not allow weddings during the entire period of the Fast of the Nativity. Since this is a non doctrinal issue, we can have different national practices. However, since we are all now in one country, we are trying to find a way to reconcile these disciplinary differences.
We are working to form a united American Orthodox Church. All canonical Eastern Orthodox Bishops in the United States are members of the Assembly of Canonical Orthodox Bishops of North and Central America, which has various committees to establish common practices and is working to form a united American Orthodox Church.