Nestorius, archbishop of Constantinople, objected to calling Mary Theotokos, 'God-bearer,' preferring Christotokos, 'Christ-bearer.' His opponents, led by Cyril of Alexandria, understood this to divide Christ into two persons loosely joined — the man Jesus and the divine Word — rather than confessing one person who is both God and man.
Theotokos
The council affirmed Theotokos. The point was never a claim about Mary's origin of the divine nature — no one taught that. The point was about Christ: the one whom Mary bore is a single person, and that person is God the Son. To deny the title was, in the council's judgment, to deny that the one born, who suffered, and who died was truly God with us.
The Outcome
Ephesus confessed the unity of Christ's person. The council's reception was contested, and its aftermath was bitter; the Church of the East, which honored the theologians Nestorius had appealed to, separated in the years that followed. Theologos describes that separation on the relevant tradition pages — here the council is recorded for what it confessed: one Christ, one person, truly God and truly born of Mary.