Hebrews 3:19

¹⁷ And with whom was God 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 never enter His rest? Was it not to those who disobeyed?

So we see that it was because of their unbelief that they were unable to enter.

Do read the surrounding verses too, because the context often says more than a single verse.

Compare

BSB: So we see that it was because of their unbelief that they were unable to enter.
KJV: So we see that they could not enter in because of unbelief.
WEB: We see that they weren’t able to enter in because of unbelief.
ASV: And we see that they were not able to enter in because of unbelief.
DARBY: And we see that they could not enter in on account of unbelief;)
BLB: And we see that they were not able to enter because of unbelief.

Jude 1:5
Although you are fully aware of this, I want to remind you that after Jesus had delivered His people out of the land of Egypt, He destroyed those who did not believe.
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John 3:36
Whoever believes in the Son has eternal life. Whoever rejects the Son will not see life. Instead, the wrath of God remains on him.”
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Mark 16:16
Whoever believes and is baptized will be saved, but whoever does not believe will be condemned.
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John 3:18
Whoever believes in Him is not condemned, but whoever does not believe has already been condemned, because he has not believed in the name of God’s one and only Son.
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1 John 5:10
Whoever believes in the Son of God has this testimony within him; whoever does not believe God has made Him out to be a liar, because he has not believed in the testimony that God has given about His Son.
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2 Thessalonians 2:12
in order that judgment may come upon all who have disbelieved the truth and delighted in wickedness.
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