And if thou say in thy heart. This exception has reference to the prohibition, which we have already noticed in this same chapter. God, in His appointment of Prophets to be His substitutes in teaching the people, had invested them with no common authority, enjoining obedience to be paid to their precepts. But those, upon whom the office of teaching is conferred, do not alwayduly fulfill it; and hence the doubt naturally arises, how the people shall determine when they are addressed as by Gods mouth, so as to distinguish the true from the false. There is, therefore, no question expressed in words, but God anticipates the secret scruples which might otherwise keep men's minds in suspense; for to "say in the heart" is equivalent to doubting in one's self when any danger is perceived. Now, to remove this difficulty, He does not enumerate all the marks of distinction; because He does not indeed allude to doctrine, but only to prophecies. But, speaking popularly, as to evil and ignorant persons, He commands them to observe whether those, who pretend to the name of Prophet, foretell the truth. But although, as we have lately seen, false teachers also rival in this respect the servants of God, and are found to be veracious in some particular prophecy; yet it is sufficient as the touchstone of their truth, to set down what happens for the most part, (epi to polu;) just as God himself chooses to be distinguished and separated from idols by the same test. Nor does Moses affirmatively pronounce that credit must be given to prophets, whenever events correspond with their predictions; but only admonishes them, that, if they consider attentively, they cannot be deceived, because God will speedily expose the false prophets to ridicule, and will confound their folly. Thus Jeremiah prudently accommodates this passage to the circumstances of his own times, in order that it may be manifested how rashly and falsely Hananiah spoke in promising impunity for those sins over which God's vengeance impended. (Jeremiah 28:6.) In sum, Moses means nothing more than that the people would not be exposed to the danger of going astray, if they endeavored heartily to obey God; because it would come to pass by His just judgment, that the temerity of all who have falsely abused His holy name should be made to appear, and thus they should be unmasked.

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?