Theophylact of Ohrid
Catena Aurea · Markusevangelium 14
And as they did eat, Jesus took bread, and blessed, and brake it, and gave to them, and said, Take, eat: this is my body. And he took the cup, and when he had given thanks, he gave it to them: and they all drank of it. And he said unto them, This is my blood of the new testament, which is shed for many. Verily I say unto you, I will drink no more of the fruit of the vine, until that day that I drink it new in the kingdom of God.
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.