Charles Ellicott Commentary John 13:17

"If ye know these things, blessed are ye if ye do them." — John 13:17 (ASV)

If you know these things, happy are you if you do them.—The first clause of this verse assumes their knowledge of the things which He had been teaching them (John 13:13–17). They were, indeed, old lessons taught before in word, and now taught in act and word.

The second clause makes their blessedness depend upon their combining action with knowledge. They had known the truth before, but their knowledge had not profited them, and they needed on this very day to be taught them again.