Theophylact of Ohrid — as excerpted in the Catena Aurea on Mark 14:22-25

“That, namely, which I now give and which ye take. But the bread is not a mere figure of the Body of Christ, but is changed into the very Body of Christ. For the Lord said, The bread which I give you is my flesh. But the flesh of Christ is veiled from our eyes on account of our weakness, for bread and wine are things to which we are accustomed, if however we saw flesh and blood we could not bear to take them. For this reason the Lord bending Himself to our weakness keeps the forms of bread and wine, but changes the bread and wine into the reality of His Body and Blood.”