The primary goal of the Council of Chalcedon of 451 was the trial of Dioscorus. Not for heresy, but for hubris, for overstepping his bounds, for using monks as thugs to beat a clergyman to death. Papal supremacy did not exist in 449 when Dioscorus flouted all tradition and continued on with the Second Council of Ephesus after the legates from Rome had left. It didn’t exist weeks later when he tried to excommunicate Leo himself. Which meant it also didn’t exist for Pope Dioscorus. They were both Archbishops in the World Church, and Dioscorus was set to lose that.
The trial of Dioscorus was interrupted by a debate about the nature of Christ. The council read aloud Leo’s Tome, a letter written by Pope Leo to Flavian; this letter was not allowed to be read at Ephesus I or II. In the Tome Leo explains the Church’s position that Christ has two natures, but is not of or from two natures. Parts of the letter worried the council as being too Nestorian, so they formed a committee to debate if the Pope was a heretic. They determined unanimously that he was not, largely by checking it against Cyril’s Twelve Chapters, which became accepted orthodoxy in the same step. Content this wasn’t heresy, they returned to the trial.
Dioscorus did not appear for his trial, despite being summoned three times. Three was the punishable number of non appearances, for which you could be automatically deposed. In Dioscorus’s case there was a lot testimony from witnesses against him – some probably exaggerated for effect, but too numerous to be entirely fabricated. The council deposed him, and banished him, and declared all his decrees from the Second Council of Ephesus null. He lived out his final days in island exile. In the Coptic and other Oriental Orthodox faiths, Pope Dioscorus is a Saint.
The council asked every bishop assembled to sign Leo’s Tome in agreement with his doctrine on Christ’s nature, but 13 Egyptian bishops refused. So the council set about writing a new creed of faith on the matter. There were four powerful non-Arian sees or church districts at that time: Antioch, Alexandria, Rome, and Constantinople. The communities of Antioch and Alexandria hated each other: Cyril v. Nestorius had been just one match in a long war. Rome was the home of St. Peter’s church, but Constantinople was the heart of the empire and its happening location made it in many ways more important.
Somehow these bickering factions worked out the Confession of Chalcedon, a long profession of faith of a True Christian as they defined it: Someone who conceived of Jesus as “truly God, and truly man… in two natures, inconfused, unchangeably, indivisibly, inseparably.” The Alexandrians from Egypt thought this confession of two distinct natures was the Nestorian heresy and still would not sign. All the rest of the East and West did. The six separate denominations making up the Oriental Orthodoxy left the ecumenical church because they believed Jesus had two united natures. In 1984 Pope John Paul II and Syrian Patriarch Ignatius Zakka Iwas released a joint statement blaming the past schism on cultural differences and language barriers, not true distinctions in theology.