A few posts ago I quoted from McGuckin how St Cyril of Alexandria let what he believed about Christ shape how he believed everything else–not a bad method, mind you. Scholars like Sergii Bulgakov pointed out that Cyril really wasn’t a brilliant thinker. He did have moments of sheer greatness, but he wasn’t an Athanasius or an Augustine. Perhaps. But he did stay doggedly to his few premises and carried the argument out, and it was this doggedness that prevented him from falling into heresy and ultimately led the Church to the Council of Ephesus.
I am quoting from Jaroslav Pelikan’s summary of how Cyril viewed the connection between Eucharist and Christology. First some background: Cyril’s opponents, the Nestorians, affirmed in one way or another that instead of two natures of Christ in one person, there were two persons of Christ (or more precisely, two centers of subjectivity in Christ). Cyril’s whole work is to refute that. Cyril maintains 1) that the divine Logos assumed human nature (alongside his divine nature) and 2) from this the Logos operates with a single source of subjectivity. What that means is the Logos determines the actions committed. In other words, we don’t see the man Jesus eating food and the divine person Jesus raising the dead on the other hand. Jesus isn’t schizophrenic.
That’s the context, now on to Pelikan.
The difference between Theodore and Cyril was that Theodore didn’t base a christology upon this Eucharistic doctrine, but Cyril did. The key to Cyril’s christological interpretation of sacramental theology lay in his emphasis upon the life-giving and transforming power of salvation in Christ, a power conveyed by the sacraments, especially the Eucharist…His proof text for the doctrine of Eucharist was not the account of its institution in 1 Corinthians, but the sixth chapter of the gospel of John: Unless you eat the flesh of the Son of man and drink his blood, you have no life in you.” These words meant, he said, that Christ “as God makes us alive, not merely by granting us a share in the Holy Spirit, but by granting us in edible form the flesh he assumed.
Commenting on these same words elsewhere, he insisted that the body given in the Eucharist could not be life-giving unless it had become the very flesh of the Logos who gives life to everything. The body received in the Eucharist was like a vivifying seed, by which the communicant was intimately joined with the Logos himself and made to be like the Logos, immortal and incorruptible.
Pelikan I: 237-238
The Lecture Hall of Tyrannus 