council · 431 · Ephesus
Some 200 bishops under Cyril of Alexandria met at Ephesus in 431. Against Nestorius they upheld that Mary is rightly called Theotokos — bearer of God — deposed him from Constantinople, received Cyril's Twelve Anathemas, and in canon 8 secured Cyprus its independence from Antioch. The Church of the East never accepted the deposition; the first lasting parting followed.
The Council of Ephesus was a council of Christian bishops convened in Ephesus in AD 431 by the Roman emperor Theodosius II. This third ecumenical council, an effort to attain consensus in the church through an assembly representing all of Christendom, confirmed the original Nicene Creed, and condemned the teachings of Nestorius, Patriarch of Constantinople, who preferred that the Virgin Mary be called Christotokos, "Christ-bearer" over Theotokos, "God-bearer"; in contrast to Cyril of Alexandria who deemed that Theotokos is enough on its own. It met from 22 June to 31 July 431 at the Church of Mary in Ephesus in Anatolia.
Nestorius' preaching against calling Mary Theotokos — one Christ or two sons?
Mary confessed Theotokos, for the child she bore is God the Word; Nestorius deposed; Cyril's Twelve Anathemas received; Cyprus autocephalous (canon 8).
Wikipedia · Catholic Encyclopedia — Council of Ephesus · OrthodoxWiki — Third Ecumenical Council · The Seven Ecumenical Councils — Ephesus (NPNF II/14)