King James Version (KJV) Authorized Version 1611

Hebrews 3:21 KJV

"And to whom sware he that they should not enter into his rest, but to them that believed not?"

Hebrews Chapter 3, Verse 21

Hebrews 3:21 in Context (Verses 18–19 KJV)

And to whom sware he that they should not enter into his rest, but to them that believed not?

So we see that they could not enter in because of unbelief.