The council of 431 is the least edifying of the ancient councils and among the most consequential. Nestorius, patriarch of Constantinople, had objected to calling Mary Theotokos, God-bearer, preferring Christotokos, on the ground that a creature cannot be the source of divinity. Cyril of Alexandria read this as dividing Christ into two subjects, a man in whom God dwelt rather than God made flesh, and pressed the case at Rome and at court.
When the bishops assembled at Ephesus, Cyril did not wait. The Syrian bishops sympathetic to Nestorius were delayed on the road, and Cyril opened the council without them, over the protest of the imperial commissioner, condemned and deposed Nestorius within a day. The Syrians arrived, constituted a council of their own, and deposed Cyril in turn. Both parties appealed to the emperor, who initially arrested both and eventually confirmed Cyril's side, partly through the persuasion of substantial gifts distributed at court, which Cyril's own correspondence records.
The doctrinal outcome was the confirmation of Theotokos and the deposition of Nestorius, and two years later a Formula of Reunion patched up relations between Cyril and the Syrian bishops on wording that conceded something to each. The procedural outcome was a demonstration that a council could be won by arriving first, which the next generation did not forget.