John Chrysostom — as excerpted in the Catena Aurea on John 6:60-71

“(Hom. xlvii. 3) Peter however having said, We believe, our Lord excepts Judas from the number of those who believed: Jesus answered them, Have not I chosen you twelve, and one of you is a devil? i. e. Do not suppose that, because you have followed Me, I shall not reprove the wicked among you. It is worth enquiring, why the disciples say nothing here, whereas afterwards they ask in fear, Lord, is it I? (Matt. 26:22) But Peter had not yet been told, Get thee behind Me, Satan; (Mat. 16:23) and therefore had as yet no fear of this sort. Our Lord however does not say here, One of you shall betray Me, but, is a devil: so that they did not know what the speech meant, and thought that it was only a case of wickedness in general, that He was reproving. The Gentiles on the subject of election blame Christ foolishly. His election does not impose any necessity upon the person with respect to the future, but leaves it in the power of His will to be saved or perish.”