In his Grace and Christology in the Early Church (Oxford Early Christian Studies) , Douglas Fairbairn argues that the Christololgical debates of the fifth century were also debates about the nature of grace.  Is grace only an assisting power that enables us to cooperate with God (Nestorian) or is grace God’s self-gift, the indwelling of the Spirit, incorporation into Christ, and engrafting into the perichoretic communion of the Trinity (Cyrillian)?  Different conceptions of salvation were implied.  As Fairbairn puts it, the question was “whether grace consisted of Christ giving the Christian power, aid, and assistance in reaching the perfect human condition or whether God gave the believer participation in his own immortality and incorruption.”

Nestorianism thus underwrote a synergistic soteriology; since the divine and human were kept at some distance, and the human nature acted somewhat autonomously from the divine, salvation was achieved through the work of the graced man Jesus, and the cooperative redemption achieved by Jesus + God became a model for human cooperation with grace.  Fairbairn again: For Nestorians, “God the Logos gives that man the power and cooperation he needs to be our pioneer in the march to the perfect age.”  Cyril, by contrast, understood grace as “God’s giving himself to humanity.”  That self-gift to man begins from the first instant of Adam’s existence, when the Creator breathed the Spirit into his nostrils, but this initial contact with the Spirit was to reach fulfillment in the indwelling of the Spirit of Christ.  ”Salvation is receiving God and not simply something from God.”