In 431 the church met in Ephesus to take up Nestorius’s explanation of the union between Christ’s humanity and his deity. As history teaches us, this is a complex topic, one which took years—hundreds and thousands of them—for the church to work through. Nesotorius’s Christology was eventually rejected because of his emphasis on the distinction between Christ’s two natures and because of his emphasis on the humanity of Christ.
Between 431 and 451 in the person of Eutyches, the head of a monastery outside of Constantinople, there arose another attempt to reconcile the complexity of Christ as the God-man. If Cyril of Alexandria emphasized the deity of Christ and if Nestorius emphasized the humanity of Christ, Eutyches merged them together, emphasizing the union of the two natures to the point that there was only one new nature after the incarnation. This union created a third nature, a brand new one, that was not particularly human or divine. For Eutyches it was not appropriate to speak of Christ’s divine nature or his human nature, or to refer to either of those to understand elements of what we read about in Scripture about him. Rather, in the incarnation something altogether new was created. This position came to be known as Monophysitism. Mono=one and the Greek word physis meaning nature.
In 451 the church met again to take up this topic: what was the relationship between the two natures of Christ and to tease out some of the complexities of the reality of God becoming human. This meeting took place in Chalcedon and is today referred to as the fourth ecumenical council of the church. Below is the statement the church agreed upon which came to be known as the Definition of Chalcedon. It reaffirmed the commitments of Nicaea and Ephesus and set some important boundary lines for the way we speak and think about Christ’s two natures. The Definition is as follows:
Following, then, the holy Fathers, we all unanimously teach that our Lord Jesus Christ is to us One and the same Son, the Self-same Perfect in Godhead, the Self-same Perfect in Manhood; truly God and truly Man; the Self-same of a rational soul and body; co-essential with the Father according to the Godhead, the Self-same co-essential with us according to the Manhood; like us in all things, sin apart; before the ages begotten of the Father as to the Godhead, but in the last days, the Self-same, for us and for our salvation (born) of Mary the Virgin Theotokos as to the Manhood; One and the Same Christ, Son, Lord, Only-begotten; acknowledged in Two Natures unconfusedly, unchangeably, indivisibly, inseparably; the difference of the Natures being in no way removed because of the Union, but rather the properties of each Nature being preserved, and (both) concurring into One Person and One Hypostasis; not as though He were parted or divided into Two Persons, but One and the Self-same Son and Only-begotten God, Word, Lord, Jesus Christ; even as from the beginning the prophets have taught concerning Him, and as the Lord Jesus Christ Himself hath taught us, and as the Symbol of the Fathers hath handed down to us.