Deuteronomy 18:21 Meaning: How Shall We Know?
You may say in your heart, “How shall we know the word which Yahweh has not spoken?”
And if thou say in thine heart, How shall we know the word which the Lord hath not spoken?
And if thou say in thy heart, How shall we know the word which Jehovah hath not spoken?
`And when thou sayest in thy heart, How do we know the word which Jehovah hath not spoken? --
And if thou say in thy heart, How shall we know the word that Jehovah hath not spoken?
Meaning in one sentence
Learn how to distinguish true from false prophecy according to Deuteronomy 18:21, 22, and why fulfilled predictions alone are not enough.
Context
Moses is nearing the end of his life, and Israel is about to enter the land. In Deuteronomy 18 he gives instructions about the kind of leaders they will need, judges, kings, priests, and prophets. He has just promised that God will raise up a prophet like him (v. 15), but he also warns sternly against anyone who speaks in God’s name without being sent (v. 20). Before he gives the practical test in the next verse, he puts into words the doubt that would surely rise in honest hearts: how can we tell when a message is not from God? This verse captures that honest, necessary question.
Key Words
Three Hebrew words carry the weight here. The phrase “say in your heart” uses amar with lebab, not an outward question to a friend, but the inner conversation you have when you are alone with your doubts. God invites that inward struggle rather than pushing it aside. The word “know” is yada, which means more than intellectual recognition. It is the kind of knowing you gain through experience and careful attention, the way a shepherd knows each sheep. And “word” is dabar, which can also mean a matter, a thing, an event. So the question is not merely about a sentence someone spoke; it is about discerning whether a whole claim truly comes from the Lord. These words together show that God expects us to think carefully, not to guess.
How Christians Read This Verse
Christians have read this verse alongside Deuteronomy 13:1, 3, which warns that a false prophet may perform signs and still lead people astray. Barnes’ Notes explains that fulfilled prophecy alone is not enough to certify a prophet, what they teach must also be tested against God’s revealed truth. Calvin agrees, saying God gives the people a workable test, but it is meant for ordinary folks who may not grasp deeper doctrine. Across traditions, there is agreement: a true prophet’s message will match what God has already shown of Himself, and it will come to pass. The two tests work together.
A common misunderstanding
Many assume that if someone says something that later happens, the message must be from God. In fact, Deuteronomy 13 shows that false prophets can get predictions right. The one sure sign that a message is not from God is when it contradicts what He has already commanded, especially drawing people away from worshiping Him alone.
Living This Out
This week you may hear someone claim to speak for God, in a sermon, a podcast, a conversation with a friend. You do not have to accept it blindly or reject it with fear. You can quietly ask: does this match what Scripture already teaches? Does it draw me closer to Christ or lead me toward something else? The answer may take time to see, but God does not leave you without a way to know. He gave His people a test, and He gives you the same patience and discernment today.
Main takeaway: God gives us practical ways to test whether a message truly comes from Him, so we are never left without guidance.
For Families
For ages 5, 8
God gives us ways to know if something is from Him. If someone says God told them something, we can check if it matches the Bible. That helps us stay safe.
For ages 9, 12
In Bible times, people had to tell true prophets from false ones. God gave a test: if a prediction didn’t come true, that prophet was false. But He also warned that even a true prediction could come from a false prophet if the message led people away from God. So we need both: does it happen, and does it honor God? God wants us to use our minds and trust His Word.
Questions People Ask
What does “say in your heart” mean in this verse?
It means talking to yourself silently, inside your own thoughts. The Hebrew phrase pictures an honest private doubt. God does not scold the question, He answers it by giving a clear test in the next verse.
Why is a fulfilled prophecy not enough to prove a true prophet?
Deuteronomy 13:1, 3 warns that a false prophet can perform signs and wonders. The deciding factor is whether the prophet’s message leads people toward the one true God or away from Him. A true prophet’s words will both come to pass and align with God’s commands.
How can Christians apply this test today?
When someone claims to have a word from the Lord, you can compare what they say with Scripture. If it contradicts what God has already revealed, it is not from Him. Also watch for the fruit over time: does their teaching draw people to Christ and produce humility, love, and obedience?
Does this verse mean all prophecy has to predict the future?
Not exactly. In context, the test refers to predictions that could be observed. But prophecy often includes teaching and warning as well. The principle is that God’s word is true and consistent, so a false message will eventually show itself, either by failing to happen or by contradicting what God has already said.
What is the difference between a prophet and a false prophet in Deuteronomy?
A true prophet is raised up by God and speaks only what God commands. A false prophet either speaks presumptuously in God’s name without being sent, or speaks in the name of other gods. The test in verse 22 gives God’s people a clear way to tell the difference.
For pastors & researchers
Hebrew interlinear, 8 commentaries & word study