St. Cyril of Jerusalem: "The Bread and Wine of the Eucharist"

● For as the Bread and Wine of the Eucharist before the invocation of the Holy and Adorable Trinity were simple bread and wine, while after the invocation the Bread becomes the Body of Christ, and the Wine the blood of Christ, so in like manner such meats belonging to the pomp of Satan, though in their own nature simple, become profane by the invocation of the evil spirit.

● Even of itself the teaching of the blessed Paul (1 Cor 11:23) is sufficient to give you a full assurance concerning those Divine Mysteries, of which having been deemed worthy, you are become of the same body and blood with Christ. For you have just heard him say directly, “That Our Lord Jesus Christ in the night in which he was betrayed, took bread, and when He had given thanks He broke it, and gave to His disciples, saying, ‘Take, eat, this is My Body’; and having taken the cup and given thanks, He said, ‘Take, drink, this is my Blood’” (Mt 26:26ff.). Since then He Himself declared and said of the Bread, ‘This is My Body,’ who shall dare to doubt any longer? And since He has affirmed and said, ‘This is My Blood,’ who shall ever hesitate, saying, that it is not His Blood?

~St. Cyril of Jerusalem: Catechetical Lectures, 19:7; 22:1.

Last Supper, Tree of Life and Four Miracle Scenes, by Taddeo Gaddi. 

Fresco, c. 1360; Refectory, Santa Croce, Florence.