Nestorius called the unity of the person of Christ into question during the fifth century, and among his most ardent opponents was Cyril, bishop of Alexandria. Cyril’s many letters to Nestorius calling for him to recant his position and explaining that Christ is one person in whom are perfectly united two natures would lay the foundation for the Council of Chalcedon in 451, which gave us the definitive statement on these matters.
In his confrontation with Nestorius, Cyril vigorously asserted the appropriateness of the title theotokos for Mary the mother of Jesus. Although some people translate theotokos as “mother of God,” the title literally means “God-bearer.” Cyril insisted on this title not because he was trying to say something about Mary but to speak primarily about Jesus and who He is. Cyril, understanding that our salvation requires a divine person to assume a human nature, was saying that the child conceived in Mary’s womb was the preexistent Son of God who in the incarnation assumed a human nature to His divine person, who from eternity has possessed a divine nature.
Nestorius rejected this title, preferring Christotokos, or “Christ-bearer,” because he held that Christ was two separate persons with two separate natures, one human and one divine. Yet while Christotokos in itself is not an untrue statement, one can affirm that title and yet deny the deity of Christ. Nestorius did not intend to deny the deity of Christ, but Christotokos is not enough to preserve what the Bible says about our Savior. The later Protestant Reformers also recognized this, and men such as John Calvin agreed with Cyril that Mary is the theotokos.
In reading the correspondence between Cyril and Nestorius, it becomes clear that one of the reasons why Nestorius was afraid to affirm that Jesus is one divine person who possesses a divine nature and a human nature was that he thought it would mean that deity could undergo suffering and change. But this was never what Cyril believed, and the orthodox view of the person of Christ has never held that deity can suffer. When we say that the one person of the Son of God suffered and died, we do not mean that He experienced this suffering and change from life to death in His divine nature. Cyril, and later, Leo the Great, whose writings also influenced Chalcedon, insisted that the divine nature cannot change and therefore cannot suffer. Only a human with a human nature can do that. So, when the Son suffered, it was only according to His humanity.
It is vital for us to affirm that the divine nature is not capable of change. If God, as God, can change, then we cannot rely on His Word or on His promises. Thus, the incarnate Son of God, Jesus Christ, was incapable of suffering according to His divine nature. The suffering He endured was according to His human nature. Christ died as a man in order to pay for the sins of other human beings. But in so doing, His deity remained unchanged.