Albert Barnes Commentary John 6:70

"Jesus answered them, Did not I choose you the twelve, and one of you is a devil?" — John 6:70 (ASV)

Have not I chosen you twelve? There is much emphasis in these words. Have not I—I, the Savior, the Messiah—chosen you in mercy and in love, and therefore it will be a greater sin to betray me? Chosen. Chosen for the apostolic office and given marks of special favor, treason is therefore the greater sin. You twelve. So small a number! Out of such a multitude who follow for the loaves and fishes, it is to be expected that there should be apostates. But when the number is so small, chosen in such a manner, then it becomes necessary for everyone, however confident he may be, to be on his guard and examine his heart.

Is a devil. He has the spirit, the envy, the malice, and the treasonable designs of a devil. The word devil here is used in the sense of an enemy, or one hostile to him.