Nestorius (386-450) was born in Germanicia in the Roman Province of Syria – now Kahramanmaraş, in Turkey. He became a monk in a monastery in Antioch, capital of the province, and soon won a reputation for piety and good sermons. On that basis, in 428, Emperor Theodosius II appointed him to the newly vacant Archdiocese of Constantinople, with its even then impressive cathedral of Hagia Sophia (this was the one built by Constantine the Great; the one we know today was built over a century later by Justinian). This was the most influential position a cleric could hope for in the East, and a rosy future appeared to be Nestorius’s.
Unfortunately, he had come to believe that there were two separate and distinct persons in Christ: one human, the other Divine.
The Virgin Mary in this system could be called Mother of Christ, but not Mother of God. Preaching on this subject, he caused a riot among his congregation, who tore him from the pulpit, ejected him from Hagia Sophia, and declared “We have no bishop.” The controversy spread throughout the Eastern Church; Theodosius convened the Council of Ephesus in 431, which condemned Nestorius’s teachings.
His followers fled to Persia, and with the assistance of the Zoroastrian rulership (who executed the pro-Roman Catholicos, Babowai, to open up the office for a Nestorian candidate) took over the Church in that country.
They were avid missionaries, and by 1300, their dioceses were scattered across Asia from Persia to China and India. But the Mongol invasions destroyed most of these, and by the 17th century they were restricted to modern-day Iran and Iraq. Their descendants include the Chaldeans (in union with Rome), and the Assyrians (who deny Nestorianism). Some modern Protestants are unknowingly Nestorian, denying to Our Lady her title of Mother of God.