Marcian, at the urging of Pope Leo, convokes the Christian church’s fourth ecumenical council, the Council of Chalcedon, in 451, primarily to resolve theological disputes about the person of Jesus Christ.

The council formally condemns the so-called Robber Synod and, once again, reaffirms the Nicene Creed.

Repudiating the Monophysite emphasis on the divinity of Christ over his humanity, the council promulgates a dogmatic statement called the "Faith of Chalcedon," which describes Christ as possessing two natures, divine and human, "without confusion, without change, without division," perfectly united in a single person.

The council, like its predecessor at Ephesus, upholds the title “theotokos” as descriptive of Mary.

The council also condemns the practice known as simony (after Simon Magus, who, according to “Acts of the Apostles,” attempted to buy the gifts of the Holy Spirit for Peter), in its most common form of buying church offices.

Leo steadfastly rejects the council’s bid to raise the status of the patriarch of Constantinople and make his see second only to Rome.

Theodoret, although identified with the Nestorian opposition, is persuaded to renounce Nestorius and is recognized as orthodox.

Eutyches is condemned, deposed, and exiled.

The council’s condemnation of Eutyches’ doctrine of Monophysitism alienates the churches of Egypt, Ethiopia, Syria, Mesopotamia, and Armenia, creating dissension in the Eastern Roman Empire.