Catena Aurea on John 13:21–30: Augustine of Hippo

Not as some careless readers think, that then Judas received singly Christ’s body. For our Lord had already distributed the sacraments of His body and blood to all of them, while Judas was there, as Luke relates; and after this He dipped the sop, as John relates, and gave it to the traitor; the dipping of the bread perhaps signifying the deep dye of his sin; for some dipping cannot be washed out again; i. e. when things are dipped, in order to receive a permanent dye. If however this dipping meant any thing good, he was ungrateful for it, and deserved the damnation which followed him; And after the sop, Satan entered into him.