Deuteronomy 18:21, 22
21. And if thou say in thine heart, How shall we know the word which the Lord hath not spoken?
21. Quia si dixeris in corde tuo, Quomodo cognoscemus verbum quod non loquutus fuerit Jehova?
22. When a prophet speaketh in the name of the Lord, if the thing follow not, nor come to pass, that is the thing which the Lord hath not spoken, but the prophet hath spoken it presumptuously: thou shalt not be afraid of him.
22. Quando loquetur propheta in nomine Jehovah, et non extiterit res illa, neque evenerit: sermo est quem non loquutus est Jehova, per superbiam enim loquutus est propheta ille: ne timeas ab eo.
21. 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, (ejpi< 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.
22. The prophet hath spoken it presumptuously. He not only condemns the folly and vanity of those who advance their own inventions in the place of God's commands, but also their arrogance; since doubtless, this is impious and intolerable audacity, to set forth the offspring of man's earthly brain as if it were a divine revelation. And on this ground it is that their impiety is detestable, who fill the air with the fumes of their revelations in order to alarm the simple. Wherefore, he adds, that they should "not be afraid" of such a prophet; because, as nothing can be more arrogant than the ministers of Satan, they confidently utter their boastings, by which we may be easily moved and even overwhelmed, unless we had this buckler to protect us, viz., that their terrific noise may be safely despised. This doctrine is now-a-days very useful for us. We know how insolently the Papists boast of the Catholic Church; of the Apostolic See; how fiercely they rage in Peter's name; how impetuously they fulminate their curses and anathemas; but, when it is ascertained that whatever they put forward as revelations of the Spirit are but the empty figments of men, it will be easy to dispel those terrors which flow from this same fountain of presumption.