John Calvin Commentary Deuteronomy 18:22

"when a prophet speaketh in the name of Jehovah, if the thing follow not, nor come to pass, that is the thing which Jehovah hath not spoken: the prophet hath spoken it presumptuously, thou shalt not be afraid of him." — Deuteronomy 18:22 (ASV)

The prophet has spoken it presumptuously. He not only condemns the folly and vanity of those who advance their own inventions in place of God’s commands, but also their arrogance. For, doubtless, this is impious and intolerable audacity: to present the creations of man’s earthly mind as if they were a divine revelation.

It is on this ground that their impiety is detestable, as they fill the air with the fumes of their revelations in order to alarm the simple.

Therefore, he adds, that they should not be afraid of such a prophet. For since nothing can be more arrogant than the ministers of Satan, they confidently utter their boastings. We could easily be moved and even overwhelmed by these, unless we had this shield to protect us: namely, that their terrifying noise may be safely despised.

This doctrine is nowadays very useful for us.

We know how insolently the Papists boast of the Catholic Church and the Apostolic See, how fiercely they rage in Peter’s name, and how impetuously they fulminate their curses and anathemas. But when it is established that whatever they put forward as revelations of the Spirit are merely the empty figments of men, it will be easy to dispel those terrors that flow from this same fountain of presumption.