451 The Council of Chalcedon begins
Nothing troubled Christianity in the years after its legality like the debates over the nature of Christ. The Arian/Athansasian dispute had centred on whether Christ was an inferior creation of God or whether He was a coequal partner with the Father and Holy Spirit in the Trinity. The 325 Council of Nicaea and centuries of politics would decide in favour of the latter position.
Next up to trouble Christendom was the question of how many natures Christ possessed and how they were related. Clearly Jesus had been born a human but he was also the Son of God: how could god and man coexist in a single entity? Nestorius, archbishop of Constantinople proposed an answer that seemed to some to denigrate the deity of Christ, while the Christians of the Levant and Egypt preferred a formula where the divine nature seemed to eclipse the human. Church councils at Ephesus in 431 and 449 had not solved the problem and the Emperor Marcian was anxious that the controversy not weaken the unity of the empire. Thus a council was summoned to Chalcedon in Asia Minor and proceedings began in October 451.
The result was the Chalcedonian definition of the Incarnation: two natures, which come together into one person and one hypostasis [individual existence].
We, then, following the holy Fathers, all with one consent, teach people to confess one and the same Son, our Lord Jesus Christ, the same perfect in Godhead and also perfect in manhood; truly God and truly man, of a rational soul and body; consubstantial with the Father according to the Godhead, and consubstantial with us according to the Manhood; in all things like unto us, without sin; begotten before all ages of the Father according to the Godhead, and in these latter days, for us and for our salvation, born of the Virgin Mary, the Mother of God, according to the Manhood; one and the same Christ, Son, Lord, only begotten, to be acknowledged in two natures, inconfusedly, unchangeably, indivisibly, inseparably; the distinction of natures being by no means taken away by the union, but rather the property of each nature being preserved, and concurring in one Person and one Subsistence, not parted or divided into two persons, but one and the same Son, and only begotten God, the Word, the Lord Jesus Christ; as the prophets from the beginning [have declared] concerning Him, and the Lord Jesus Christ Himself has taught us, and the Creed of the holy Fathers has handed down to us.
As a way of ensuring unity, the Council of Chalcedon was a dreadful failure. The church leaders of Syria and Egypt refused to accept this definition and stuck to the position known as Monophysitism or miaphysitism wherein the singleness of Christ’s nature is emphasized. These eastern churches, which in two centuries would fall under Muslim rule, would grow apart from Chalcedonian Christianity and remain separate to this day.