Deuteronomy 18:21

21. if thou say in thine heart] Deu_8:17.

The Cambridge Bible for Schools and Colleges (gen. ed. J. J. S. Perowne; Cambridge University Press, 1878–1918) — public domain. Text from a community e-Sword module.

When a prophet speaketh in the name of the Lord,.... Says he comes from God, is sent by him, and has a commission from him to say so:

if the thing follow not, nor come to pass; as the prophecy of Hananiah, Jer 28:3 that is the thing which the Lord hath not spoken; or otherwise it would have come to pass, unless when a condition is either expressed or implied, as the repentance or disobedience of a people; see Jer 18:7.

but the prophet hath spoken it presumptuously; in a bold and daring manner, with great impiety and impudence, out of his own head and heart, being a mere device and imagination of his own, which, not having the fear of God, he delivered as coming from the Lord:

thou shall not be afraid of him; not only to reprove him for his wickedness, but also to punish him for it; showing no regard to the high character he assumes, nor to the great pretensions he makes to sanctity, knowledge, and familiarity with God.

Next: Deuteronomy Chapter 19

John Gill's Exposition of the Whole Bible (1746–1763) — public domain (CC PDM 1.0). Text via the HelloAO Bible API.

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 1Ki 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 Deu 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.

Albert Barnes' Notes on the Bible (1834–1885) — public domain. Text from a community e-Sword/MySword module.

Vers. 21, 22. The test by which it was to be discovered which was the true prophet and which the false, was the fulfillment or non-fulfillment of his prediction. The reference here is to the prediction of proximate events events that were to happen within a limited period, but which were notsuch as one not divinely instructed could foresee. When such came to pass, the pretensions of the prophet were thereby substantiated, and his authority established (cf. 1Sa_3:19; Joh_2:18, etc.) This was a morecertain test than such as was offered by signs and wonders. (Deu_13:2, etc.)

The Pulpit Commentary (1880–1919), ed. H.D.M. Spence-Jones & Joseph S. Exell — public domain. Text from a community e-Sword module.

וכי תאמר בלבבך AND IF THOU SAY IN THINE HEART [HOW SHALL WE KNOW THE WORD WHICH THE LORD HATH NOT SPOKEN]? — you will once ask this — when Hananiah the son of Azur (the false prophet) comes and prophesies (Jeremiah 27:16) “Behold, the vessels of the Lord’s house will now shortly be brought again from Babylon”, whilst Jeremiah (the true prophet) stands and proclaims (Jeremiah 27:19—22) “concerning the pillars, and concerning the laver, and concerning the bases, and concerning the residue of the vessels [which remain in this city which Nebuchadnezzar king of Babylon took not,…]” which did not go into exile to Babylon with Jeconiah, king of Judah, “they shall be carried to Babylon [and there they shall remain]” together with Zedekiah when he will go into exile (Sifrei Devarim 178:2).

Rashi on the Tanakh — English translations: “The Judaica Press complete Tanach with Rashi, translated by A. J. Rosenberg” (CC BY 4.0); “Pentateuch with Rashi's commentary by M. Rosenbaum and A.M. Silbermann, 1929-1934” (public domain); “The Metsudah Tanach series, Lakewood, N.J” (CC BY 4.0); and “The Book of Joshua, Metsudah Publications, 1997” (CC BY 4.0). Text via Sefaria.

V’CHI'(AND WHEN) THOU SAY IN THY HEART: ‘HOW SHALL WE KNOW THE WORD WHICH THE ETERNAL HATH NOT SPOKEN?’ “You are destined to say this when Hananiah the son of Azur [the false prophet] will come and prophesy, Behold, the vessels of the Eternal’s House shall now shortly be brought back from Babylon while Jeremiah [the true prophet] stands and proclaims concerning the pillars, and concerning the sea, and concerning the bases, and concerning the residue of the vessels, Verse 19. which were not exiled with Jeconiah, they shall be carried to Babylon, Verse 22. with the exile of Zedekiah.” This is Rashi’s language from the Sifre. 178. This interpretation [of the Sifre] is based on the fact that the verse does not state “v’im (and if) thou say in thy heart” [but it states ‘v’chi’ — (and when) thou say in thy heart]. This is as I have mentioned that the Torah tells future events through allusions. But the purport of the verse is to say, “How shall we know the word which the Eternal hath not spoken, so that we can slay the one who so prophesies?” And he commanded that if the event does not come to pass then he should be killed because the Eternal hath not spoken it. Do not fear him because of his wisdom or because of the sign that he gave [even though] the sign or the wonder did come to pass. Similarly the people could ask, “How shall we know the word which G-d had spoken, to which we have been obligated to hearken in whatever He shall command?” But this [Verse 22] is the answer to both questions, for, in the case of every prophet we should look ahead to that which he foretold. If it does not come to pass we shall know that he is a false prophet and he is to be killed, and if all that he predicted does come to pass he is established to be a prophet of the Eternal, and we are bound to hearken to him in all that he commands in the name of G-d, just as he said, unto him ye shall hearken. Even to transgress the words of the Torah, [we are required to obey him] provided that it is to meet the particular need of the time, as in the case of Elijah on Mount Carmel [who, in his attempt to discredit idolatry, brought offerings to G-d on Mount Carmel despite the fact that it was forbidden to do so outside the Sanctuary Court in Jerusalem]. This is the sense of the expression, [a prophet] like unto me, [And G-d said, I will raise them up a prophet] like unto thee, meaning that he will first be established a prophet of the Eternal either by a sign or wonder, similar to what is said [of Moses], and he did the signs in the sight of the people. And the people believed, — or that he foretells some future event and it occurs. And thus the Rabbis have said in Tractate Sanhedrin: “Whoever disobeys a prophet, is liable to death by the hands of Heaven, as it is written, I will require it of him.” And there the Rabbis asked: “And how is one to know that someone is a prophet [and that, for refusing to obey him,] he will be punished? If the prophet gave him a sign. But in the case of Micaiah. The following verse says that the man was slain by a lion because of his refusal to obey the prophet. The lesson was clear: just as that man was punished, so would Ahab be punished for disobeying the prophetic command to smite Ben-hadad. Now this certain man of the sons of the prophets is identified by tradition as having been Micaiah [a prophet mentioned there in Chapter 22 Verse 13]. When Micaiah ordered the man to smite him, he gave no sign that he was a prophet, yet the man was punished for disobeying! Hence the Gemara’s question: How was he to know that Micaiah was a prophet? the prophet gave him no sign and yet he was punished [for refusing to hearken to him! The Gemara answered]: It is different when a person is known to be an established prophet [in which case he need not give a sign]. And if this is not the case, how did Isaac hearken to Abraham on Mount Moriah [to submit himself to slaughter, a command, which had been given to Abraham but not to him]? And how did we rely on Elijah on Mount Carmel to bring offerings outside [the Sanctuary Court in Jerusalem? Obviously we must say that] it is different if the prophet is established [in which case he must be obeyed even if he does not offer a sign].”Now Rashi wrote: “They have already been commanded that if one comes to entice you from [observing] one of the commandments [of G-d] you are not to hearken to him unless it has been clearly established by you that he is an absolutely righteous man, like Elijah on Mount Carmel [who brought the offerings outside the Sanctuary Court] because of the needs of the time. Therefore it states, unto him ye shall hearken.” — But this does not appear to be correct, for the necessary verification is not that he is a righteous man but that he is a true prophet known to all by having previously foretold future events which indeed came to pass, this being his “sign” as referred to in this section, or some wonders which he has done in our presence. This constitutes the presumption of prophets. In saying “a perfectly righteous man,” perhaps Rashi meant what Harav Rabbeinu Moshe [ben Maimon] wrote in Hilchoth Yesodei Hatorah: “When a prophet is assigned a mission, he is given a sign or wonder so that the people might know that G-d had truly sent him. We do not believe everyone who shows a sign or wonder to be a prophet. Only a man whom we already know to be worthy of prophecy — by reason of his wisdom and deeds wherein he stands pre-eminent among all his contemporaries, and follows the paths of prophecy, in its sanctity and renunciation [of physical lust] — who then comes and performs a sign or wonder and says that G-d sent him, it is a commandment to hearken to him, as it is said, unto him ye shall hearken. It is possible that such a man may perform a sign or wonder and still not be a prophet [in which case] the sign must have some [other] meaning. Nevertheless, it is a commandment to listen to him, since he is a great man, learned and worthy of prophecy, we assign him the presumption [that his claim is true], just as we have been commanded to render a decision [in court] by the testimony of two proper witnesses even though it is possible that they testified falsely, because we consider them to be legitimate witnesses [having no basis to assume the contrary] we assign them the presumption of legitimacy in their testimony. On such matters it is said, The secret things belong unto the Eternal our G-d, and it is further said, for man looketh on the outward appearance, but the Eternal looketh on the heart.”

Ramban (Nachmanides) on the Tanakh — English translation: “Commentary on the Torah by Ramban (Nachmanides). Translated and annotated by Charles B. Chavel. New York, Shilo Pub. House, 1971-1976” (CC BY 4.0). Text via Sefaria.