The Council of Chalcedon was a church council held in AD 451 at Chalcedon, (modern Kadıköy in Istanbul, Turkey). The Council was called by Emperor Marcian to assert the orthodox Catholic doctrine of the two natures of Christ.
The council is numbered as the fourth ecumenical council by the Catholic Church, the Eastern Orthodox Church, and most Protestants. Oriental Orthodox Churches do not agree with the conduct and the proceedings of the Council, commonly calling it “Chalcedon, the Ominous”. This disagreement led the Oriental Orthodox Churches to separate from the rest of Christianity after the Council of Chalcedon.
Followers of the Council believe its most important achievement was to issue the Chalcedonian Definition, stating that Jesus is “perfect both in deity and in humanness; this selfsame one is also actually God and actually man.”