What does it mean to refuse to believe the gospel?
Refusing the gospel is rejecting that Christ died for one's sins, paid the sin debt, and rose from the dead in power. It is the one sin God cannot forgive, because it rejects His provision for forgiveness, sealing one's doom.
Les teaches that Israel’s problem in the wilderness was not ultimately the golden calf or their immorality, vile as those sins were; God could forgive them. Their real problem was unbelief—they could not believe what He said. The passage says they did not enter His rest because they believed not, and the last verse concludes they could not enter in because of unbelief. Les explains that nothing has changed. God can forgive to the uttermost any sin, even the vilest, except unbelief. When people refuse to believe that Christ died for them, paid their sin debt, and rose from the dead in power, and that is all He asks—believe it—then their doom is sealed.