We know ... - We know that God commanded Moses to deliver the law. In that they were correct; but they assumed their interpretation of the law to be infallible, and, hence, condemned Jesus.
As for this fellow - The word “fellow” is not in the original. It is simply “this.” The word “fellow” implies contempt, which it cannot be proved they intended to express.
Whence he is - We know not his origin, his family, or his home. The contrast with the preceding member of the sentence shows that they intended to express their belief that he was not from God. They knew not whether he was mad, whether he was instigated by the devil, or whether he spoke of himself. See John 7:27; John 8:48-52.