Early on, the church faced the heresy of docetism, which said Jesus only appeared to possess a body. His physicality was an illusion; the Son of God did not really take on our human flesh. Thankfully, full-fledged docetism did not plague the church for long, and Christians quickly recognized the importance of Jesus’ possessing a true human nature. For example, even though the Niceno–Constantinopolitan Creed (325, 381) defends the full deity of Christ, it also highlights the incarnation: the Son of God “for us and for our salvation . . . was incarnate by the Holy Spirit of the Virgin Mary, and was made man.”
Yet, docetic tendencies did not disappear entirely before the great Christological controversies of the fourth and fifth centuries. Some thinkers taught a view of the person of Christ that effectively eliminated His true humanity, even though they did not go so far as to teach that Jesus only appeared to be human. One of these heresies was Eutychianism, named for Eutyches, a fifth-century monk.
Eutyches taught that Christ possesses only one nature, that the divine nature of Christ swallows up or absorbs the human nature of Jesus, such that He is left with but one theanthropic nature (from the Greek theos, “God,” and anthrōpos, “man”). Instead of being one person with two natures, as orthodox Christology asserts, Christ is one person with one nature, according to Eutychianism.
The Eutychian heresy ends up denying the true humanity of Christ, but it also ends up denying the true deity of our Savior. If the divine nature of Christ absorbs the human nature of Christ, we are left with a composite nature that is neither truly human nor truly divine. Instead, it is a third kind of nature, a divine-human nature.
Eutychianism gives us at least two problems. First, it makes many descriptions of Jesus in Scripture misleading because the Eutychian Jesus cannot be subject to the normal, nonsinful limitations of humanity. Today’s passage, for instance, says that Jesus sometimes slept during His earthly ministry. But God does not grow weary (Isa. 40:28), so if Christ’s deity absorbs His humanity, the limitation of tiredness is overcome and Jesus was only pretending to sleep on the boat. Second, if Jesus does not possess a true human nature and a true divine nature, He cannot represent both God and man. He cannot be the perfect Mediator between the Lord and His people. Only a human being can pay for the sins of other human beings, and if Jesus does not have a true human nature, as in Eutychianism, He cannot atone for our sin.