Here are a couple of favorite texts from the fathers of the church. The first is the third anathema of Cyril of Alexandria's twelve anathemas against Nestorius. The second is paragraph seven of the third Letter to Nestorius from Cyril.
These texts are so important because they show the connection between the Eucharist and Christology a connection which Luther and Lutheran fathers shared. The Eucharist is not an addendum to the Gospel, however precious. It is the Gospel. It is Christ.
The language of the "life giving flesh" of Christ is taken from John 6. That the flesh of Christ is live-giving is a central tenet of the faith, crucial to the atonement and to the Eucharist. If Christ's flesh can secure the life of the world on the cross then his flesh is also life giving and present in the Eucharist. The central point in either case is the presence of God in created things for salvation. Incarnation is the beating heart of the crucifixion and the Supper.
The first citation is from John McGuckin's St. Cyril of Alexandria: The Christological Controversy : Its History, Theology, and Texts. The second is the Post-Nicene Fathers translation swiped from the Internet and is in the public domain.
1. If anyone does not confess that the Lord's flesh is life giving and the very own flesh of the word of God the Father but says that it is the flesh of someone else different to him, and joined to him in terms of dignity or indeed only having a divine indwelling , rather than being life-giving as we have said, because it has become the personal flesh of the Word who has power to bring all things to life, let him be anathema.
2. And of necessity will we add this too: Declaring the Death in the Flesh of the Ony-Begotten Son of God, that is Jesus Christ, and confessing His living again from the dead and His Assumption into Heaven, we celebrate the Unbloody Service in the churches, and thus approach to the Mystic Blessings, and are sanctified, rendered partakers of the Holy Flesh and Precious Blood of Christ the Saviour of us all. And not as though we were receiving common flesh (God forbid) nor yet that of a man sanctified and connected with the Word by unity of dignity, or as having a Divine Indwelling, but as truly quickening and the own Flesh of the Word Himself.
For being by Nature Life as God, since He became One with His own Flesh, He rendered it Life-giving. So that even though He say to us, Verily, verily, I say unto you, Except ye eat the Flesh of the Son of Man, and drink His blood [John 6:53], we shall not account it also as that of one of us (for how will a man's flesh be life-giving in its own nature?) but as having truly become the own Flesh of Him Who for our sakes both became and was called Son of Man.