King James Version (KJV)

Hebrews 3:18

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

Verse in context — Hebrews 3

For some, when they had heard, did provoke: howbeit not all that came out of Egypt by Moses.

But with whom was he grieved forty years? [was it] not with them that had 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?

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

Jesus as the ultimate High Priest and fulfillment of the Old Covenant.