This man is not of God - Is not sent by God, or cannot be a friend of God.
Because he keepeth not the sabbath-day - They assumed that their views of the Sabbath were correct, and by those views they judged others. It did not occur to them to inquire whether the interpretation which they put on the law might not be erroneous. Men often assume their own interpretations of the Scriptures to be infallible, and then judge and condemn all others by those interpretations.
A sinner - A deceiver; an impostor. They reasoned conclusively that God would not give the power of working such miracles to an impostor. The miracles were such as could not be denied, nor did even the enemies of Jesus attempt to deny them or to explain them away. They were open, public, frequent. And this shows that they could not deny their reality. Had it been possible, they would have done it; but the reality and power of those miracles had already made a party in favor of Jesus, even in the Sanhedrin John 7:50; John 12:42, and those opposed to them could not deny their reality. It may be added that the early opponents of Christianity never denied the reality of the miracles performed by the Savior and his apostles. Celsus, Porphyry, and Julian - as acute foes of the gospel as perhaps have ever lived - never call this in question. They attempted to show that it was by some evil influence, or to account for the miracles in some other way than by admitting the divine origin of the Christian religion, but about the facts they had no question. Were they not as well qualified to judge about those facts as men are now? They lived near the time; had every opportunity to examine the evidence; were skilful and talented disputants; and if they could have denied the reality of the miracles they would have done it. It is scarcely possible to conceive of more conclusive proof that those miracles were really performed, and, if so, then the Lord Jesus was sent by God.
A division - Greek, “A schism.” A separation into two parties.