What does the sin of unbelief consist of?
It consists of refusing to believe God's Word, especially the gospel: that Christ died for them, paid their sin debt, and rose from the dead in power. Israel's immorality was forgivable, but their unbelief kept them from entering God's rest; refusing that gospel seals a person's doom.
Les explains that Israel's immorality at the golden calf was not what God held against them; He could forgive even that vile sin. Their real problem was unbelief—they could not believe what He said. The passage says they could not enter His rest because they believed not, and the last verse concludes it was because of unbelief. Les further teaches God can forgive to the uttermost any sin except unbelief. Unbelief is refusing to believe that Christ died for them, paid their sin debt, and rose from the dead in power; that is all He asks—believe it. If a person refuses that gospel, his doom is sealed.