Although Pope Leo I, the Great, asked Theodosius II to summon a council for the definition of the orthodox doctrine once more, to bring an end to the ecclesiastical chaos, his request was refused. Shortly after Theodosius' death his sister Pulcheria, marrying a senator and veteran soldier Marcian (450-57), became empress and in accordance with the Pope's wish summoned a great council at the church of St. Euphemia in Chalcedon.
The participancy of some six hundred bishops at this council shows the extent of the displeasure that the Robbers Council had created in the eastern provinces. This was the greatest of the seven Ecumenical Councils, and in importance second to only the First Council of Nicaea. The council reconfirmed that Christ was a single person with two natures, one divine and one human.
However, it was unable to define the relationship between the two natures which was the cause of the controversy. Thus both Nestorianism, which overstressed the human element in Christ, and Monophysitism, which overemphasized the divine at the expense of the human nature of Christ, were condemned.
The result did not satisfy either Alexandria or Antioch. Among the other decisions taken at the council - when the Roman delegates were absent - was the elevation of Constantinople to the level of Rome: 'The See of Constantinople shall enjoy equal privileges with the See of Old Rome.' This left Rome nothing but titular supremacy. In other words while the bishop of Rome might enjoy a primacy of honor in the Church universal, the bishop of Constantinople, the evident capital of what was left of the Roman empire, became his equal in authority.
This canon known as 'Canon Twenty-Eight' was strongly objected to by Rome and became one of the steps which ultimately led to the separation of the Churches of the East and West in 1054. The new position given to the church of Constantinople, combined with national and political factors, also alienated Egypt, Syria and Palestine from the empire.
Shortly after the council the Egyptian Monophysites elected their own patriarch in Alexandria, separate from the one assigned to the port by the capital, and took the first step for the foundation of the Egyptian Church which would be known as the Coptic Church. When the Moslem armies who believed in the single Person of Allah arrived in the seventh century, the Coptic Church readily submitted to them.