This mostly has to do with the 1923 population exchange between Turkey and Greece where nationality was defined along lines of religion. Populations from Greece that were muslim were forced to move to Turkey and vice versa with Christians in Turkey. Just under 2.5 million people were moved (some by choice, but not always). It's interesting because the religious criteria led to a number people who could not speak their new country's language. There are still communities in Turkey where people speak Greek at home and there are neighborhoods (such as Nea Smyrni in Athens) made up of these ex-pats that still retain "foreign" qualities in their food, music, and speech.

PS I think you mean Eastern Orthodox.