John Chrysostom — as excerpted in the Catena Aurea on John 13:21-30

“(Hom. lxxii. 2.) That thou doest, do quickly, is not a command, or a recommendation, but a reproof, meant to shew too that He was not going to offer any hindrance to His betrayal. Now no man at the table knew for what intent He spake this unto him. It is not easy to see, when the disciples had asked, Who is he, and He had replied, He it is to whom I shall give a sop, how it was that they did not understand Him; unless it was that He spoke too low to be heard; and that John lay upon His breast, when he asked the question, for that very reason, i. e. that the traitor might not be made known. For had Christ made him known, perhaps Peter would have killed him. So it was then, that none at the table knew what our Lord meant. But why not John? Because he could not conceive how a disciple could fall into such wickedness: he was far from such wickedness himself, and therefore did not suspect it of others. What they thought He meant we are told in what follows: For some of them thought, because Judas had the bag, that Jesus had said unto him, Buy those things that we have need of against the feast, or, that he should give something to the poor.”