Deuteronomy 18:22 - Joseph Benson’s Commentary on the Old and New Testaments

If the thing follow not Which he gives as a sign of the truth of his prophecy. That is the thing which the Lord hath not spoken The falsehood of his prediction shows him to be a false prophet. He hath spoken it presumptuously Impudently ascribing his own vain and lying fancies to the God of truth. For though the mere fulfilling of a sign, or working of a bare miracle, was not to be considered as sufficient of itself to establish a false and wicked doctrine, as is stated Deuteronomy 13:1-3; yet, on the other hand, a man that pretended to work a miracle, or predict a future event, in confirmation of a message said to be received from Jehovah, or from some other god, and who failed in the performance of the miracle, or the thing foretold not coming to pass, evidently proved himself to be an impostor. Thou shalt not be afraid of him That is, of his predictions or threatenings, so as to be deterred thereby from doing thy duty in bringing him to deserved punishment.