As for this man, we know not whence he is. When they say so, they refer not to his country or the place of his birth, but to the prophetical office. For they allege that they have no knowledge of his calling, so as to receive him as having proceeded from God.

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.

We know not from whence he is - As if they had said: We have the fullest assurance that the commission of Moses was Divine; but we have no proof that this man has such a commission: and should we leave Moses, and attach ourselves to this stranger? No.

We know that God spoke to Moses,.... Out of the bush, and told him who he was, and sent him to deliver the children of Israel out of Egyptian bondage, and spoke the ten words, or law unto him, and by him delivered them to the children of Israel, and to whom he spake face to face, as a man does to his friend, and mouth to mouth, and not in dark sayings; they mean, they knew that Moses had his mission, commission, and credentials from God:
but as for this fellow; so they contemptuously called the Lord Jesus Christ,
we know not from whence he is; contradicting what others of them had said, John 7:27. They imagined they knew the country from whence he came, which they supposed to be Galilee, and the place where he was born, which they concluded was Nazareth; though in both they were in the wrong; and they knew his parents, Joseph and Mary, and his brethren and sisters; but as to his divine filiation, they knew nothing of it; nor would they own his mission, commission, and credentials to be from heaven; and pretended they had no reason to conclude they were.

We know not whence he is - By what power and authority he does these things.