They were not able to enter because of unbelief.

Hebrews 3 from the daily reading in the One Year Bible

Therefore, holy brethren, partakers of a heavenly calling, consider Jesus, the Apostle and High Priest of our confession; ² He was faithful to Him who appointed Him, as Moses also was in all His house. ³ For He has been counted worthy of more glory than Moses, by just so much as the builder of the house has more honor than the house. ⁴ For every house is built by someone, but the builder of all things is God. ⁵ Now Moses was faithful in all His house as a servant, for a testimony of those things which were to be spoken later; ⁶ but Christ was faithful as a Son over His house—whose house we are, if we hold fast our confidence and the boast of our hope firm until the end.

⁷ Therefore, just as the Holy Spirit says,

“Today if you hear His voice, ⁸ Do not harden your hearts as when they provoked Me,
As in the day of trial in the wilderness, ⁹ Where your fathers tried Me by testing Me,
And saw My works for forty years. ¹⁰ “Therefore I was angry with this generation,
And said, ‘They always go astray in their heart, And they did not know My ways’;
¹¹ As I swore in My wrath, ‘They shall not enter My rest.’”

The Peril of Unbelief

¹² Take care, brethren, that there not be in any one of you an evil, unbelieving heart that falls away from the living God. ¹³ But encourage one another day after day, as long as it is still called “Today,” so that none of you will be hardened by the deceitfulness of sin. ¹⁴ For we have become partakers of Christ, if we hold fast the beginning of our assurance firm until the end, ¹⁵ while it is said,

“Today if you hear His voice, Do not harden your hearts, as when they provoked Me.”

¹⁶ For who provoked Him when they had heard? Indeed, did not all those who came out of Egypt led by Moses? ¹⁷ And with whom was He angry for forty years? Was it not with those who sinned, whose bodies fell in the wilderness? ¹⁸ And to whom did He swear that they would not enter His rest, but to those who were disobedient? ¹⁹ So we see that they were not able to enter because of unbelief.

It was not God’s intention to have the Israelites wander in the wilderness for forty years. After taking them out of Egypt God took them to the Promised Land. It was because of their unbelief, that they listened to the bad report of the spies rather than to the promise of God, that they did not enter God’s rest in the Promised Land. The writer of Hebrews exhorts us to believe so that we, unlike the rebellious Israelites, can enter God’s rest and all that He promises for us. The text says:     Take care, brethren, that there not be in any one of you an evil, unbelieving heart that falls away from the living God. But encourage one another day after day, as long as it is still called “Today,” so that none of you will be hardened by the deceitfulness of sin. For we have become partakers of Christ, if we hold fast the beginning of our assurance firm until the end, while it is said, “Today if you hear His voice, Do not harden your hearts, as when they provoked Me.” For who provoked Him when they had heard? Indeed, did not all those who came out of Egypt led by Moses? And with whom was He angry for forty years? Was it not with those who sinned, whose bodies fell in the wilderness? And to whom did He swear that they would not enter His rest, but to those who were disobedient? So we see that they were not able to enter because of unbelief.     We need to recognize that those who wandered in the wilderness those forty years still enjoyed the provision of God. They were still given what they needed, food and water each day, from the hand of God. God is a good God. He still provides even if we harden our hearts and resist or rebel against Him. In Matthew 5:43-45 Jesus says:    “You have heard that it was said, ‘You shall love your neighbor and hate your enemy.’ But I say to you, love your enemies and pray for those who persecute you, so that you may be sons of your Father who is in heaven; for He causes His sun to rise on the evil and the good, and sends rain on the righteous and the unrighteous. “   Some might say then, what advantage is there to following God rather than seeking pleasure. Previously, in Matthew 5:19-20, Jesus said:   Whoever then relaxes one of the least of these commandments and teaches men so, shall be called least in the kingdom of heaven; but he who does them and teaches them shall be called great in the kingdom of heaven. For I tell you, unless your righteousness exceeds that of the scribes and Pharisees, you will never enter the kingdom of heaven.   While God causes the sun to shine and provides rain for all, obedience to God, following Him and not rebelling against His way, is the way to enter into His kingdom and His rest.

God will not leave us even if we have rebelled. His desire is that none would be lost. 2 Peter 3:9 says:  The Lord is not slow about His promise, as some count slowness, but is patient toward you, not wishing for any to perish but for all to come to repentance.  God is good, patient and kind, even to those who rebel against Him. Romans 2:4 says:  Do you think lightly of the riches of His kindness and tolerance and patience, not knowing that the kindness of God leads you to repentance?   God does not provide for the unrighteous because He embraces or condones their behavior. God shows them kindness and patience so that they might repent. The writer exhorts us saying:   Take care, brethren, that there not be in any one of you an evil, unbelieving heart that falls away from the living God. But encourage one another day after day, as long as it is still called “Today,” so that none of you will be hardened by the deceitfulness of sin. For we have become partakers of Christ, if we hold fast the beginning of our assurance firm until the end, while it is said, “Today if you hear His voice, Do not harden your hearts, as when they provoked Me.”   

So, I encourage you today, do not be hardened by the deceitfulness of sin. Do not relax the commandments of God, so that you may enter His kingdom and His rest; so that you will have not only what you need, but you will have all that you desire. Psalm 37:1-9 says:   Do not fret because of evildoers, Be not envious toward wrongdoers. For they will wither quickly like the grass And fade like the green herb. Trust in the Lord and do good; Dwell in the land and cultivate faithfulness. Delight yourself in the Lord; And He will give you the desires of your heart. Commit your way to the Lord, Trust also in Him, and He will do it. He will bring forth your righteousness as the light And your judgment as the noonday. Rest in the Lord and wait patiently for Him; Do not fret because of him who prospers in his way, Because of the man who carries out wicked schemes. Cease from anger and forsake wrath; Do not fret; it leads only to evildoing. For evildoers will be cut off, But those who wait for the Lord, they will inherit the land.       I say: Amen, thank You heavenly Father, Lord Jesus Christ and precious Holy Spirit.  Amen.