In order to provide some background material for the Orientale Lumen Conferences in 2010, with the help of Father Michael Hayduk, I prepared a summary of the first Seven Ecumenical Councils that I post here.

SITE: Ephesus (S. of Smyrna in SW Asia Minor).
YEAR: A.D. 431
POPE: St. Celestine I, 423 – 432
EMPEROR: Theodosius II, 408 – 450
ACTION: Called by the Eastern Emperor, Theodosius II, influenced by his pious sister, St. Pulcheria (Emperor in the West was Valentinian III, 425 – 455), and ratified by Pope Celestine I, this council condemned the heresy of Nestorius by clearly defining the Divine maternity of the Blessed Virgin Mary. There are two natures in Christ (Divine and Human), but only one Person (Divine). Mary is the Mother of this one Divine Person, the eternal Second Person of the Blessed Trinity. Nestorius was deposed as bishop of Constantinople. This council also briefly affirmed the condemnation of the Pelagians (see local Council of Carthage, A.D. 416).
NOTE: St. Cyril of Alexandria, Doctor of the Church (d.444), was the bishop presiding.
HERESIARCH: NESTORIUS.

The Council of Ephesus, of more than 200 bishops, presided over by St. Cyril of Alexandria representing Pope Celestine I, defined the true personal unity of Christ, declared Mary the Mother of God (theotokos) against Nestorius, Bishop of Constantinople, and renewed the condemnation of Pelagius.