If any man will do his will
Meaning, not one that perfectly fulfils the law, which is the good, and perfect, and acceptable will of God; for there is no man that does this, or can do it; nor is it so said here, "if any man do his will", but "if any man will do" it; that is, is desirous of doing it; who has it wrought in him both to will and do, of the good pleasure of God, by his grace and Spirit; with whom to will is present, though, he has not power to perform, and so is a spiritual man; and who believes in the Lord Jesus Christ, which is one branch of the will of God; and who depends upon the Spirit and grace of God, and acts from a principle of love to God, and in the exercise of faith on Christ:
he shall know of the doctrine, whether it be of God, or
speak of myself;
not a man of mere natural knowledge and learning, or a man of theory and speculation, is a judge of doctrine; but he that leans not to his own understanding, and implores the assistance of the Divine Spirit, and who is for reducing doctrine into practice: he knows by the efficacy of the doctrine upon his heart, and the influence it has on his life and conversation; by its coming not in word only, but in power; and by its working effectually in him, whether it is divine or human, of God or of man.