Deuteronomy 18:21 - Albert Barnes' Notes on the Bible

And if thou say in thine heart, How ... - The passage evidently assumes such an occasion for consulting the prophet as was usual among the pagan, e. g., an impending battle or other such crisis (compare 1 Kings 22:11), in which his veracity would soon be put to the test. Failure of a prediction is set forth as a sure note of its being “presumptuous.” But from Deuteronomy 13:2 ff we see that the fulfillment of a prediction would not decisively accredit him who uttered it: for the prophet or dreamer of dreams who endeavoured on the strength of miracles to seduce to idolatry was to be rejected and punished. Nothing therefore contrary to the revealed truth of God was to be accepted under any circumstances.