"To the divinity is truly united the body that was born of the Blessed Virgin, not in the sense that the body which ascended into heaven comes down again, but in the sense that the bread and wine are changed into God's body and blood. If you ask how this takes place, it is sufficient for you to know that it is done by the Holy Spirit, in the same way that our Lord took flesh from the Holy Mother of God and made it subsist in Himself. We apprehend and understand no more than that God's word is true and efficacious, and can do all things. But the way it was done is simply beyond our powers of investigation. This much may be said without misgiving: just as bread by eating, and wine and water by drinking, are naturally changed into the body and blood of him who eats and drinks, and do not become a body different from the living body that existed before, so the bread that had previously been placed on the table, and also the wine and water, are converted into the body and blood of Christ by the invocation and coming of the Holy Spirit, in a manner that exceeds the powers of nature. Once this action has been performed, there are no longer two substances, but one and the same."
-St. John Damascene in De fide orthodoxa.