6 “Simon Peter therefore beckons, and says to him.” The expression is noteworthy, as indicating that something was said not by any sound of words, but by merely beckoning with the head. “He beckons, and says;” that is, his beckoning is his speech. For if one is said to speak in his thoughts, as Scripture says, “They said [reasoned] with themselves;” how much more may he do so by beckoning, which expresses outwardly by some sort of signs what had previously been conceived within!
What, then, did his beckoning mean? What else but that which follows? “Who is it of whom He speaks?” Such was the language of Peter's beckoning; for it was by no vocal sounds, but by bodily gestures, that he spoke. “He then, having leaned back on Jesus' breast,”— surely the very bosom of His breast this, the secret place of wisdom!— “says unto Him, Lord, who is it? Jesus answered, He it is to whom I shall give a piece of bread, when I have dipped it. And when He had dipped the bread, he gave it to Judas Iscariot, the son of Simon.
And after the bread, Satan entered into him.” The traitor was disclosed, the coverts of darkness were revealed. What he got was good, but to his own hurt he received it, because, evil himself, in an evil spirit he received what was good. But we have much to say about that dipped bread which was presented to the false-hearted disciple, and about that which follows; and for these we shall require more time than remains to us now at the close of this discourse.
Source: Tractates on the Gospel of John (New Advent)