John Lightfoot on John 13:26

[And when he had dipped the sop.] This was a very unusual thing, to dip a sop and reach it to any one: and what could the rest of the disciples think of it? It is probable they took it as if Christ had said to Judas, "What thou doest, do quickly: do not stay till the supper be done and the tables withdrawn; but take this sop to make up your supper, and begone about the business you are to despatch." So they might apprehend the matter; only John, indeed, understood what it meant: unless perhaps Peter, being not ignorant of the question John asked our Saviour, might not be ignorant of what Christ answered him by that action.