If the thing follow not; which he gives as a sign of the truth of his prophecy. He means the prediction of some strange and wonderful event, as appears by comparing this with Deuteronomy 13:1,2. The Lord hath not spoken: the falsehood of his prediction shows him to be a false prophet, though the truth and accomplishment of his prediction had not proved him to be a true prophet, as is evident from Deuteronomy 13:2,3. Presumptuously; impudently ascribing his own vain and lying fancies to the God of truth. Thou shalt not be afraid of him, i.e. of his predictions or threatenings, so as to be scared from doing thy duty in bringing him to deserved punishment.
Related Commentaries of Deuteronomy 18:22
Deuteronomy 18:22
22 When a prophet speaketh in the name of the LORD, if the thing follow not, nor come to pass, that is the thing which the LORD hath not spoken, but the prophet hath spoken it presumptuously: thou shalt not be afraid of him.