1 Corinthians 10:5

Nevertheless, with most of them God was not pleased; for they were overthrown in the wilderness.

1 Corinthians 10:11

Now these things happened to them as an example, and they were written down for our instruction, upon whom the ends of the ages have come.

Hebrews 3:19

So we see that they were not able to enter because of unbelief.

Treasury of Scripture Knowledge

Summary

I the LORD have said, I will surely do it unto all this evil congregation, that are gathered together against me: in this wilderness they shall be consumed, and there they shall die.

Bible References

This evil

1 Corinthians 10:5
Nevertheless, with most of them God was not pleased; for they were overthrown in the wilderness.
Hebrews 3:19
So we see that they were not able to enter because of unbelief.