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: Chalcedon, (north of Constatinople)
YEAR: A.D. 451
POPE: St. Leo I, the Great, 440 – 461
EMPEROR: Marcian, 450 – 457
ACTION: Called by Emperor Marcian, spouse of the chaste and noble St. Pulcheria, and ratified by Pope St. Leo the Great, the council condemned the heresy of the Abbot Eutyches, MONOPHYSITISM, which claimed that there existed only “one nature” (the divine) in Christ from the Incarnation onward. Though the council had approved the assertion that Constantinople should be ranked first after Rome ecclesiastically, Pope St. Leo did not. The primacy of the See of Rome was due to it’s possession of the Chair of Peter, not to any political power. In his “Dogmatic Epistle,” read by his legates at the end of the second session of the council (Oct. 10, 451), Pope St. Leo I also declared invalid all that had been done at the “Robber Synod of Ephesus” (a false Ephesus II): ” ….we see no Council, but a den of thieves (Latrocinium).” In the greatest testimony of the Eastern Council to the primacy of the Pope, the bishops cried out: “Behold the faith of the fathers, the faith of the Apostles; thus through Leo has Peter spoken!” Eutyches was excommunicated.
NOTE: Pope St. Leo I, Doctor of the Church (d. 461), was called the “Soul” of Chalcedon.
HERESIARCH / HERETICS: EUTYCHES – MONOPHYSITES.

The Council of Chalcedon — 150 bishops under Pope Leo the Great and the Emperor Marcian — defined the two natures (Divine and human) in Christ against Eutyches, who was excommunicated.