8 October – 10 November 451

The Fourth Ecumenical Council (Chalcedon)

After the death of Theodosius II, the new rulers Marcian and Pulcheria convened an ecumenical council. It opened at Chalcedon on 8 October 451. It was the largest gathering of the ancient Church, with 500 to 630 fathers.

The council overturned the decisions of the 449 "Robber Council." It deposed Dioscorus of Alexandria in absentia, condemning him not for his faith but for insubordination. It also rehabilitated the victims of 449 – Theodoret of Cyrus, Ibas of Edessa, and others.

The fathers recognized the Nicene and Constantinopolitan creeds and Cyril of Alexandria's letters as a sufficient basis of faith. Even so, after prolonged disputes, they adopted a new Definition on 22 October. The disputes notably concerned whether Leo the Great's "Tome" was compatible with Cyril's teaching.

The Definition proclaimed one Christ, perfect in Godhead and perfect in manhood, consubstantial with the Father in Godhead and with us in manhood. He is acknowledged "in two natures" without confusion, without change, without division, without separation, united in one Person and one Hypostasis.

Canon 28 placed the Patriarch of Constantinople on an equal honor with Rome. This provoked a sharp protest from the papal legates and Pope Leo himself.

The Definition became the norm for the West and most of the East. But it was rejected by a substantial part of the East (Egypt, Syria, Armenia), giving rise to a lasting Monophysite schism.