The Rum-Orthodox Church dates back to the apostles Peter and Paul, who initiated the foundation of churches in Antioch, a city in today's southeast Turkey. A part of these Antiochian Christians did not accept the decisions of the ecumenical council of Chalcedon (451), while the other part followed the council and was later called "Rum-Orthodox". Thus the Rum-Orthodox Church belongs to the community of the Byzantine Orthodox Churches, but is in dialogue with the Oriental Orthodox Churches. Head of the Rum-Orthodox Church since 2012 is Yuhanna X. Today there are about 10,000 Rum-Orthodox Christians living in Germany, about half of whom are organized churchly. They are served by two auxiliary bishops.

Further information can be found here:

http://rum-orthodox.de/