Because of unbelief: they refused to believe God's promise that He would drive out the Canaanites.
The generation that came out of Egypt saw God's works for forty years, yet continued to test Him and harden their hearts. At Kadesh Barnea they said, "We can't take the land," even though God said He would clear it for them. As Hebrews 3:19 says, "they could not enter in because of unbelief."
Hebrews 3:19 — This verse directly states that the reason Israel could not enter God's rest was their unbelief.
Hebrews 3 describes the wilderness generation that provoked God in the day of testing. The Holy Spirit warns, "To day if ye will hear his voice, harden not your hearts, as in the provocation." These people tested and proved God and saw His works for forty years, yet God said, "They do alway err in their heart; and they have not known my ways." Therefore He sware in His wrath, "They shall not enter into my rest." Hebrews 3:16-19 then asks who heard and provoked: was it not all that came out of Egypt by Moses, but with whom was He grieved forty years? It was with them that sinned, whose carcases fell in the wilderness. And to whom sware He that they should not enter into His rest, but to them that believed not? The conclusion is explicit: "So we see that they could not enter in because of unbelief."
Les Feldick emphasizes that Israel's failure was rooted in unbelief, not primarily in the outward sins people might expect. He notes that while the Israelites had been guilty of gross immorality, such as the golden calf incident, the writer of Hebrews attributes their exclusion to unbelief. God had promised to drive out the Canaanites, clear the land, and hand it over to them ready to enjoy—vineyards, orchards, houses, everything. Instead of believing, Israel demanded spies, which was never God's idea. When the spies returned, the majority said, "Can't do it," and the people wept. Only Joshua and Caleb believed God, and they were the exceptions. The rest of that generation was kept out and their carcasses fell in the wilderness. Les applies this directly to Christians today: the problem in Christianity is still unbelief—people will not simply say, "I believe what God says." The warning is to not harden your heart as in the provocation.
Instead of moving in to take the land as God commanded, Israel made Moses send spies. God had already told them He would clear the land and drive out the Canaanites. But when the twelve spies returned, ten brought back an evil report, and the people wept and said they could not take the land—they were like grasshoppers and the cities were walled. God said, "I'll give it to you," yet they refused to enter.
Les acknowledges that Israel had been guilty of much immorality, including the golden calf just months earlier. But he points out that Hebrews attributes their failure to unbelief, not their immorality. The oath to keep them out was sworn to them that believed not, and the cause is stated as unbelief.
Scripture says they tested and proved God and saw His works for forty years. During all that time God was grieved with that generation, saying they always erred in their heart and did not know His ways. So He swore in His wrath that they would not enter His rest.
Les applies the warning directly: the problem with Christianity today is unbelief. People simply will not break down and say, "I believe what God says." The scriptural warning is to harden not your hearts as in the provocation.
Israel Wept Instead of Believing
Israel wept at Tisha B'Av because they refused to believe God's promise to clear the land; their unbelief provoked God to swear they would not enter His rest.
Unbelief Led Israel to Demand Spies
God had promised to drive out the Canaanites and hand the prepared land to Israel, but their unbelief made them demand spies—something God never instructed.
Only Joshua and Caleb Believed
Nearly the whole generation that came out of Egypt was in unbelief; only Joshua, Caleb, Moses, and Aaron were exceptions, while the rest fell in the wilderness.
Majority Was Wrong at Kadesh Barnea
The ten spies were the majority but dead wrong; Caleb and Joshua were right, and the people's refusal to believe God kept them out of His rest.
Rejecting God's Promise at Kadesh Barnea
Israel rejected God's promise by saying they were like grasshoppers and could not defeat the Canaanites, even though God had already sent hornets ahead of them.
Unbelief, Not Immorality, Kept Them Out
Paul refers to the wasted forty years and says Israel did not enter because of their unbelief—not their immorality, though they had been guilty of much immorality.
Harden Not Your Hearts
Les uses Hebrews 3:15 to show the ongoing lesson of Kadesh Barnea: do not harden your heart as in the provocation.
The Land Was Ready for Their Rest
The land of milk and honey was completely ready, requiring no labor or battle, but Israel's unbelief caused them to miss what God had prepared.
Hebrews 3:19: So we see that they could not enter in because of unbelief.
This is the direct scriptural conclusion explaining why Israel failed to enter the Promised Land.
Hebrews 3:18: And to whom sware he that they should not enter into his rest, but to them that believed not?
The oath excluding Israel was specifically made to those who did not believe.
Hebrews 3:17: But with whom was he grieved forty years? was it not with them that had sinned, whose carcases fell in the wilderness?
God's grief over forty years was directed toward the sinners whose unbelief caused their death in the wilderness.
Hebrews 3:15: To day if ye will hear his voice, harden not your hearts, as in the provocation.
This warning connects the wilderness provocation to the present-day need to respond in faith.
Hebrews 3:10-11: Wherefore I was grieved with that generation, and said, They do alway err in their heart; and they have not known my ways. So I sware in my wrath, They shall not enter into my rest.
God's verdict on the wilderness generation: they continually erred in heart and would not enter His rest.
Scripture concludes in Hebrews 3:19 that they could not enter because of unbelief. Les emphasizes that while the Israelites were also guilty of immorality, the causative issue was their refusal to believe what God had promised.
They demanded spies when God had already told them to go in and take the land. When ten spies reported giants and walled cities, they wept and said they couldn't take the land, even though God said He would drive out the Canaanites.
Joshua and Caleb, along with Moses and Aaron, believed God. The rest of that generation was kept out and their carcasses fell in the wilderness because of their unbelief.
Les says the problem in Christianity today is unbelief—people will not simply believe what God says. The warning is to not harden your heart as in the provocation.
Watch the Teaching: Why Did Israel Reject God? Romans 10:18-21 Explained