Nestorius became Patriarch of Constantinople in 428 and soon objected publicly to preachers in his own church calling Mary Theotokos, God-bearer, a title already in devotional use for at least two centuries. His concern was philosophical rather than a slight against Mary, since he worried the word implied that the eternal, impassible God could be born from a woman or could suffer and die on a cross, which struck him as theologically incoherent. He proposed Christotokos, Christ-bearer, as a safer alternative, understanding Christ's divine and human natures as joined closely but remaining in some sense two distinct subjects.
Cyril, Patriarch of Alexandria, saw in this a fatal splitting of Christ into two persons rather than one, and pressed the argument that if Mary bore only the human Jesus and not God himself in a single undivided person, then Christ's saving work could not properly be called God's own act. The dispute escalated into open ecclesiastical warfare, drawing in the emperor Theodosius II, until a council convened at Ephesus in 431 CE, dominated by Cyril's supporters, condemned Nestorius and his teaching and formally upheld Theotokos as correct usage. Nestorius was deposed and exiled, and communities loyal to his Christology broke away, forming what became the Church of the East, sometimes called Nestorian, which persists today, chiefly in Iraq, Iran, India and their global diaspora communities.