Its story
Nestorius, a monk of Antioch, became patriarch of Constantinople in 428. Trained in the Antiochene tradition, which was attentive to Christ's full humanity, he objected to calling Mary Theotokos ('she who gave birth to God'), because the term seemed to him to confuse the natures or make God someone born in time; he proposed Christotokos ('mother of Christ'). His teaching spoke of a 'conjunction' (synapheia) of the Word with the assumed man in one prosōpon.
Cyril of Alexandria saw this as dividing Christ into two sons and defended the 'hypostatic union': the same eternal Word is the subject who is born, suffers and dies in his flesh. Pope Celestine backed Cyril, and the Council of Ephesus (431), convened by Theodosius II and led by Cyril, deposed Nestorius before the Antiochene bishops under John of Antioch arrived; they held their own synod. The split was healed by the Formula of Reunion (433). Nestorius died in exile in Egypt around 451. Later, the Second Council of Constantinople (553) condemned the 'Three Chapters' (Theodore of Mopsuestia and certain writings of Theodoret and Ibas).
From the late 19th century, the discovery of a work by Nestorius preserved in Syriac, the Book of Heracleides, led many scholars to ask whether Nestorius really held the 'two sons' doctrine attributed to him, or whether he was partly a victim of terminological misunderstanding and of rivalry between the sees of Alexandria and Constantinople. Other historians judge that even so his Christology failed to express a real unity. The debate remains open.
The Church of the East (in Persia, outside the Roman Empire) honors Nestorius and Theodore as teachers but rejects the name 'Nestorian', which it regards as an external and misleading label, and does not recognize itself in the doctrine condemned at Ephesus. In 1994 Pope John Paul II and Patriarch Mar Dinkha IV of the Assyrian Church of the East signed a Common Christological Declaration in which both sides recognized that they profess the same faith in Christ, true God and true man, though with different formulas.