All sin, of course, is bad and, if continued in, can harden the heart; but the sin which the writer has in mind here, as the context makes abundantly clear, is the sin of unbelief, the sin of hearing the gospel but rebelling against it (3:16), the sin of refusing to enter the promised land, for which God was angry with Israel for forty years (3:17); the sin of disobedience and unbelief (3:18–19). Do notice that all those who came out of Egypt by Moses were guilty of this rebellion,