Commentaries:
God foresees the inevitable appearance of false prophets. He knows His people can become easily confused at the frightening words of any charismatic, clever-talking, Scripture-twisting, self-professed prophet. God also knows that His children will ask how they can know if a prophet's words are truly sent by Him (verse 22).
Staff
The Prophet
In the remaining verses of the chapter, verses 20-22, the subject matter changes. God seems to acknowledge that He has purposely left the true Prophet unnamed, perhaps wanting His people to put effort into recognizing the Prophet or prophets who truly represent Him. Of course, He wants us to use His criteria, which He proceeds to explain. This is just as true today as it was in ancient Israel.
Staff
The Prophet
If a man falsely claims to speak in God's name, or if he speaks in another god's name, he is worthy of death. If the man's predictions do not occur, he is a false prophet. Conversely, if a man speaks in God's name, and what he says happens, he may indeed be a true prophet (Jeremiah 28:8-9).
Apart from Christ Himself, Ezekiel may be the clearest case of a true prophet. He prefaces many of his prophecies with "the word of the LORD came to me, saying . . ." (Ezekiel 3:16; 6:1; etc.), followed by a direct quotation of God's words. This is speaking "a word in My name" (Deuteronomy 18:20). If it is indeed what God commanded him to say, he is guiltless, whether or not it comes to pass within his lifetime. Many of Ezekiel's prophecies, for instance, had a near fulfillment (in type) and a far fulfillment (antitype). In both cases, he is shown to be a true prophet of God.
Richard T. Ritenbaugh
Was Herbert Armstrong a False Prophet?
Other Forerunner Commentary entries containing Deuteronomy 18:21:
Jeremiah 28:5-9
John 13:19