John Calvin Commentary Deuteronomy 18:21

"And if thou say in thy heart, How shall we know the word which Jehovah hath not spoken?" — Deuteronomy 18:21 (ASV)

And if you say in your heart. This exception refers 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, commanding that obedience be paid to their precepts.

But those upon whom the office of teaching is conferred do not always duly fulfill it; therefore, the doubt naturally arises how the people are to determine when they are addressed as by God's 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 people's minds in suspense; for to “say in the heart” is equivalent to doubting within oneself 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, when addressing evil and ignorant persons, He commands them to observe whether those who claim to be Prophets 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 consider what happens for the most part (ἐπὶ τὸ πολὺ), just as God Himself chooses to be distinguished and separated from idols by the same test.

Nor does Moses affirmatively declare that prophets must be believed 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 applies this passage to the circumstances of his own times, so that it might be shown 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 earnestly endeavored 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 would be made apparent, and thus they would be unmasked.