John Lightfoot on Matthew 26:22

[Lord, is it I?] The very occasion, namely, eating together and fellowship, partly renews the mention of the betrayer at the Paschal supper; as if he had said, "We are eating here friendly together, and yet there is one in this number who will betray me": partly, that the disciples might be more fully acquainted with the matter itself: for at the supper in John 13, he had privately discovered the person to John only; unless perhaps Peter understood it also, who knew of John's question to Christ, having at first put him upon it by his beckoning. The disciples ask, Is it I? partly through ignorance of the thing, partly out of a sincere and assured profession of the contrary.