Christological heresies affect our salvation because the diminishment of Christ makes our salvation uncertain. If Christ is less than divine, then He accomplishes a less than divine salvation, that is, a purely human salvation. Then a huge burden is placed on us humans. If Jesus accomplishes only a salvation consisting of human merit, then His merit is merely His own and cannot serve us and give us salvation. We are abandoned to following the example of his merely human merit by producing enough of it for our own purposes. We are left truly naked and alone in the presence of God with a burden we cannot bear (Gal 2:15-16).

John Cassian (c. 360 - 435) pointed out that Nestorianism revived the older heresy of Pelagianism. Pelagius (d. c. 418), a British monk, argued that humans had the spiritual resources in them to fulfill the law and will of God. Pelagianism was primarily a heresy of salvation because it suggested that man was capable of saving himself with sufficient moral effort. But this heresy of salvation also called into question the character and meaning of the death of Jesus. Why does He need to die if we are equipped with the moral aptitude to satisfy the law on our own? If we are uncertain why He needs to die, we are easily led to be uncertain who it is that died. If moral persons get right with God by dint of their own ethical aptitude, then why shouldn't a purely human Christ do likewise? The value of the life and death of the Messiah then is entirely exemplary; by seeing His sacrifice and piety we are encouraged in our moral efforts. Again we are left to our own devices. And if we are candid with ourselves, we will conclude that the case is hopeless and despair over our salvation.

Nestorianism (c. 428) then returned the element of despair to the theme of salvation by splitting the person of Christ into two persons; one human and one divine, who work in concert by a spiritual Vulcan mind meld, rather than a real personal union. In the true personal union, one person consisted of two complete natures, divine and human. For the Nestorians, the person of Christ was not the God-Man, but a two-headed hydra, a god and a man, in consultation at best or at odds at worst. The works of the man were the works of the man. The works of God were the works of God. The works of man would not be salvific, because they would be purely human works. The works of God would not benefit humanity, because the human would not participate in them. This leads back to the acute problem of salvation for us humans, for we are again left to our own devices as we have diminished the divine and human wholeness of the person of Christ. A diminished Christ diminishes salvation.

John Cassian