The Council of Chalcedon was held at Chalcedon, city of Bithynia, which is now a district of Kadikoy, on the Asian side of Istanbul. At the time of the Fourth Council Bithynia was an independent city, separated from Constantinople by the Bosphorus Strait.
The judgements and definitions of divine nature issued by the council marked a significant turning point in the Christological debates that led to the separate establishment of the church in the Western Roman Empire during the 5th century. The Council of Chalcedon issued the Chalcedonian Definition, which repudiated the notion of a single nature in Christ, and declared that he has two natures in one person and hypostasis. It also insisted on the completeness of his two natures: Godhead and manhood. The council also issued 27 disciplinary canons governing church administration and authority. In a further decree, later known as the canon 28, the bishops declared the See of Constantinople (New Rome) second only in honour and authority to Rome.
Although Pope Leo I the Great asked Theodosius II to summon a council for the definition of the orthodox doctrine once again, to put an end to the ecclesiastical chaos, his request was denied.
Shortly after Theodosius' death his sister Pulcheria, married to a senator, veteran soldier and Orthodox Christian Marcian (450-457), the next Emperor, had become an Empress and surrendering to the Pope's demand summoned a great council at the church of St. Euphemia in Chalcedon though Leo had pressed for it to take place in Italy. Initially, the Emperor Marcian called for it to convene at Nicaea; however the Huns invasions forced it to be moved at the last moment to Chalcedon.
The participation of some six hundred bishops in this council shows the extent of the displeasure that the Robbers Council had created in the eastern provinces. This was the greatest of the first seven Ecumenical Councils, and in importance, second only to 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 in the absence of the Roman delegates was the elevation of Constantinople to the level of Rome.
This left Rome nothing but titular supremacy. In other words, while the bishop of Rome might enjoy a primacy of honour 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 reasons that ultimately led to the separation of the Churches of the East and West in 1054. The new status given to the church of Constantinople, together 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 as opposed to the one assigned to the port by the capital, and took the first step to establish the Egyptian Church which would become known as the Coptic Church. When the Moslem armies which believed in the single Person of Allah arrived in the seventh century, the Coptic Church readily submitted to them.