Docetism, Eutychianism, and Apollinarianism differed on their views of the two natures of Christ, but they agreed upon the unity of His person. To put it another way, regardless of how they viewed the human nature and its relationship to the divine nature, they taught that there was but one subject who acted when Christ acted. Christ is not two subjects or two persons, one human and one divine, who do different things. Rather, He is one subject, one personal agent who speaks and acts no matter what is said or done.
For all of their errors regarding the natures of Christ, these heresies did understand that Christ is but one person. The Nestorian heresy, on the other hand, not only confessed two different natures in Christ but also two different persons. Named after Nestorius, the fifth-century bishop of Constantinople, Nestorianism was the final major heresy that eventually gave rise to the church’s definitive response regarding the person of Christ at the Council of Chalcedon in 451.
According to Nestorius, Jesus is the union of two persons—a human person and a divine person. This is not a union of essences but rather a close moral union. In other words, Nestorius believed the union was not such that we could say the humanity of Jesus actually belongs to the Son of God. Instead, it belongs only to the human person. When Christ died, it was not the incarnate Son of God suffering according to His human nature; it was the human person who died. When Christ performed a miracle, it was not the incarnate Son of God acting according to His divine nature to manifest His power; it was the divine Logos acting independently of the human person in Jesus.
The errors of Nestorianism become evident when we reflect on the atonement. If Christ is two persons, who died on the cross? It cannot be the infinite divine person of the Son, for He has not assumed a human nature. He possesses only a divine nature, which cannot experience suffering. So, it must have been the human person who suffered and died because the human person in Christ has a human nature, which can experience suffering. But then we have the death only of a finite person, for human persons are finite. And the merit of a finite human sacrifice could hardly be applied to anyone besides the finite person who offers it. Thus, the Westminster Larger Catechism 38 says that Christ had to be God—He had to be a divine person with a human nature so as to give His human suffering sufficient worth to atone for many (Heb. 5:9). Nestorianism gives us an insufficient atonement.