Hebrews 3:19

3:16For who were the ones who heard and rebelled? Were they not all those Moses led out of Egypt?

3:17And with whom was God angry for forty years? Was it not with those who sinned, whose bodies fell in the wilderness?

3:18And to whom did He swear that they would never enter His rest? Was it not to those who disobeyed?

3:19So we see that it was because of their unbelief that they were unable to enter.