John 6:67 - Matthew Poole's English Annotations on the Holy Bible

It is probable that some stayed besides the twelve, for it is said only that many of his disciples turned back. Nor was our Saviour (who knew the hearts of all) ignorant what they would do; but he had a mind both to try them by this question, and also to convince them that there was a false brother amongst them, whose wickedness (though it lay hid from them) would in a short time discover itself.