Hebrews 3:19

‘And we see that they were not able to enter in because of unbelief.’

Note the sequence. They provoked (Hebrews 3:16), they sinned (Hebrews 3:17), they were disobedient (Hebrews 3:18), they were guilty of unbelief (Hebrews 3:19). Their hearts became harder and harder. And thus they could not enter into God’s rest (Deuteronomy 12:9; Exodus 33:14). To enter God’s rest was to be settled in the land and delivered from surrounding enemies (Deuteronomy 3:20; Deuteronomy 25:19; Joshua 1:15; Joshua 21:44; Joshua 22:4; Joshua 23:1).

So the lessons up to this point are on the danger of being entrapped by sin and allowing it to develop within; and the danger of assuming that the majority is always right; and the danger of disobedience and unbelief; and the danger of turning away from God’s appointed deliverer; all of which in Israel’s case had resulted in God’s judgment. All these will cause us to fail to enter into His rest, a rest obtained by becoming partakers in Christ (Hebrews 3:14).