The Creed of Chalcedon (or Definition) was adopted at the fourth and fifth sessions of the fourth œcumenical Council, held at Chalcedon, opposite Constantinople, A.D. 451 (Oct. 22d and 25th). It embraces the Nicæno-Constantinopolitan Creed, and the christological doctrine set forth in 30the classical Epistola Dogmatica of Pope Leo the Great to Flavian, the Patriarch of Constantinople and martyr of diophysitic orthodoxy at the so-called Council of Robbers (held at Ephesus in 449).

While the first Council of Nicæa had established the eternal, pre-existent Godhead of Christ, the Symbol of the fourth œcumenical Council relates to the incarnate Logos, as he walked upon earth and sits on the right hand of the Father. It is directed against the errors of Nestorius and Eutyches, who agreed with the Nicene Creed as opposed to Arianism, but put the Godhead of Christ in a false relation to his humanity. It substantially completes the orthodox Christology of the ancient Church; for the definitions added during the Monophysite and Monothelite controversies are few and comparatively unessential. As the Nicene doctrine of the Trinity stands midway between Tritheism and Sabellianism, so the Chalcedonian formula strikes the true mean between Nestorianism and Eutychianism.