The body which is born of the holy Virgin is in truth body united with divinity, not that the body which was received up into the heavens descends, but that the bread itself and the wine are changed into God's body and blood. But if you enquire how this happens, it is enough for you to learn that it was through the Holy Spirit, just as the Lord took on Himself flesh that subsisted in Him and was born of the holy Mother of God through the Spirit. And we know nothing further save that the Word of God is true and energises and is omnipotent, but the manner of this cannot be searched out. But one can put it well thus, that just as in nature the bread by the eating and the wine and the water by the drinking are changed into the body and blood of the eater and drinker, and do not become a different body from the former one, so the bread of the table and the wine and water are supernaturally changed by the invocation and presence of the Holy Spirit into the body and blood of Christ, and are not two but one and the same.
[St. John Damascene, An Exposition of the Orthodox Faith, Book IV, Chapter 13.]