Could do nothing - Could do no such work as this. This reasoning was conclusive. The fact that Jesus could perform miracles like this was full proof that he was commissioned by God - proof that never has been and never can be refuted. One such miracle proves that he was from God. But Jesus gave many similar proofs, and thus put his divine mission beyond the possibility of doubt.

If this man were not of God, etc. - A very just conclusion: God is the fountain of all good: all good must proceed from him, and no good can be done but through him; if this person were not commissioned by the good God, he could not perform such beneficent miracles as these.

If this man were not of God,.... If he had not his mission, commission, and credentials from God; if he had not been sent by him, and had not authority from him, and was not assisted by him, as man, or God was not with him,
he could do nothing; or "not do these things", as the Syriac version reads; that is, such miraculous works; or, as the Persic version, "he could not do this miracle": open the eyes of a man born blind. His doing this is a full proof that he is of God, and comes from him.

He could do nothing - Of this kind; nothing miraculous.