Monophysitism is the belief that Christ does not possess two distinct natures: divine and human, but only one nature which is a fusion of the two. This new nature is a tertium quid or third thing that results from combining the two in the incarnation. This means Christ is not human in the exact same way we are or God in the same way the Father is. The result is that Christ is neither truly God or truly man at the same time. Therefore, in an attempt to be consistent with Scripture, monophysitists must emphasize one nature over the other (since there is no distinction between them) and historically this has resulted in docetism or the belief that Christ is not fully human. Eutyches of Constantinople, for example, argued that the human nature of Christ was swallowed up or dissolved into his divine nature which took preeminence. Monophysitism developed as an overreaction to Nestorianism. But in attempting to combat Nestorianism which spoke of Christ as if he were two persons, monophysitists failed to maintain the distinction between Christ’s humanity and deity.
A key verse for understanding the person of Christ is Colossians 2:9: “For in him the whole fullness of deity dwells bodily.” Paul distinguishes between Christ’s “deity” and his humanity when he speaks of the “body” of Christ. The one who is Christ is both fully divine and has a true human body. Hebrews 2:17 says that Christ “had to be made like his brothers in every respect.” He had to become human exactly as we are or else he could not save us. If he does not share the same nature we do as humans, then he could not have been our substitute on the cross. Therefore, he must possess a human mind, emotions, soul, and will to be truly human. As the church father Gregory of Nazianzus declared, “That which He has not assumed, He has not healed” (Against Apollinarius, Letter 101). All that we are he had to become so that we would become like him through the renewing of the image of God (Rom 8:29).
Monophysitism is also impossible because there can be no change in the divine nature for God to be immutable. Jesus Christ with respect to his divine nature “is the same yesterday and today and forever” (Heb 13:8). If Christ’s divinity merged with his humanity to create a new nature, then God himself changed rather than understanding the incarnation as the addition of humanity to the person of Christ who is God. The Council of Chalcedon sets forth the biblical view of Christ as one person existing in two natures:
“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”
Those who advocate monophysitism today prefer to use the term miaphysitism to distinguish what they believe from the docetism of Eutyches of Constantinople. Modern advocates of this belief include the Coptic, Ethiopic, and Syriac Orthodox churches. These are known as the Oriental Orthodox and claim Cyril of Alexandria as a defender of their views who spoke of the “one nature of the Word of God incarnate.” But neither Cyril of Alexandria or the Council of Chalcedon speak with the authority of Scripture. The Scriptures are sufficient for answering the question of who Jesus is and they declare that he exists both “in the form of God” and “in human form” at the same time (Phil 2:6-7).