Origen

Or, He spoke generally, to prove the nature of each of their hearts, and to evince the wickedness of Judas, who would not believe in One who knew his heart. I suppose that at first he supposed that the thing was hid from Him, deeming Him man, which was of unbelief; but when he saw that his heart was known, he embraced the concealment offered by this general way of speaking, which was shamelesness. This also shews the goodness of the disciples, that they believed Christ’s words more than their own consciences, for they began each to say, Lord, is it I? For they knew by what Jesus had taught them that human nature is readily turned to evil, and is in continual struggle with the niters of the darkness of this world; (Eph. 6:12.) whence they ask as in fear, for by reason of our weakness the future is an object of dread to us. When the Lord saw the disciples thus alarmed for themselves, He pointed out the traitor by the mark of the prophetic declaration, He that hath eaten bread with me hath wantonly overthrown me. (Ps. 41:9.)