428–431 (council – June 431); union of 433 (established by 437–438)

The Nestorian controversy and the Third Ecumenical Council of Ephesus (431); the Union of 433

Nestorius, an Antiochene monk, became bishop of Constantinople in 428. Following Theodore of Mopsuestia, he rejected calling Mary "Theotokos" (God-bearer), preferring "Christotokos." He spoke of a mere "conjunction" (synapheia) of the two natures. In effect this made them two persons joined only by a moral union.

Cyril of Alexandria was backed by Rome (Pope Celestine). He insisted on the "hypostatic union" of the Word and the flesh. In his third letter he issued twelve anathemas against Nestorius.

Convened by Theodosius II, the council opened at Ephesus in June 431. Cyril did not wait for the Antiochene bishops and condemned and deposed Nestorius. John of Antioch arrived days later and convened a rival council. It in turn deposed and excommunicated Cyril. The Western legates sided with Cyril.

The emperor arrested both sides but ultimately upheld the rulings of Cyril's council. Nestorius was exiled and died in Egypt. There, under a pseudonym, he wrote the apologetic "Book of Heraclides."

Prolonged negotiations ended in 433 with the "Formula of Reunion." This Antiochene confession presented Christ as perfect God and perfect man, consubstantial with the Father and with us, with Mary as Theotokos. Cyril accepted it, to the dismay of extreme Alexandrians (future Monophysites) and some Antiochenes.

Irreconcilable Nestorians withdrew to eastern Syria and Persia. There a separate Church of the East took shape.