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.

And if thou say in thine heart, &c. Such a thought arises in the mind, and it appears to be a difficulty, and a query is made upon it:

how shall we know the word which the Lord hath not spoken? What marks, signs, and criterions are those by which it may be known that it is not a word that comes from the Lord?

The false prophet was to be discovered by the fact, that the word proclaimed by him did not follow or come to pass, i.e., that his prophecy was not fulfilled. Of him they were not to be afraid. By this injunction the occurrence of what had been predicted is made the criterion of true prophecy, and not signs and wonders, which false prophets could also perform (cf. Deuteronomy 13:2.).

And if thou say in thine heart, How shall we know the word which the LORD hath not spoken?

18:15-22 It is here promised concerning Christ, that there should come a Prophet, great above all the prophets; by whom God would make known himself and his will to the children of men, more fully and clearly than he had ever done before. He is the Light of the world, Joh 8:12. He is the World by whom God speaks to us, Joh 1:1; Heb 1:2. In his birth he should be one of their nation. In his resurrection he should be raised up at Jerusalem, and from thence his doctrine should go forth to all the world. Thus God, having raised up his Son Christ Jesus, sent him to bless us. He should be like unto Moses, only above him. This prophet is come, even JESUS; and is He that should come, and we are to look for no other. The view of God which he gives, will not terrify or overwhelm, but encourages us. He speaks with fatherly affection and Divine authority united. Whoever refuses to listen to Jesus Christ, shall find it is at his peril; the same that is the Prophet is to be his Judge, Joh 12:48. Woe then to those who refuse to hearken to His voice, to accept His salvation, or yield obedience to His sway! But happy they who trust in Him, and obey Him. He will lead them in the paths of safety and peace, until He brings them to the land of perfect light, purity, and happiness. Here is a caution against false prophets. It highly concerns us to have a right touchstone wherewith to try the word we hear, that we may know what that word is which the Lord has not spoken. Whatever is against the plain sense of the written word, or which gives countenance or encouragement to sin, we may be sure is not that which the Lord has spoken.