| the second ecumenical council in 381 which added wording about the Holy Spirit to the Nicene Creed | a 6th century masterpiece of Byzantine architecture in Istanbul; built as a Christian church, converted to a mosque in 1453, and made into a museum in the middle of the 20th century |
| the fifth ecumenical council in 553 which held Origen's writings to be heretic | |
| the sixth ecumenical council in 680-681 which condemned Monothelitism by defining two wills in Christ, divine and human | |
| the council in 869 that condemned Photius who had become the patriarch of Constantinople without approval from the Vatican, thereby precipitating the schism between the eastern and western churches | |
| the largest city and former capital of Turkey; rebuilt on the site of ancient Byzantium by Constantine I in the fourth century; renamed Constantinople by Constantine who made it the capital of the Byzantine Empire; now the seat of the Eastern Orthodox Church | |