What is the only sin God cannot forgive?
The only sin God cannot forgive is unbelief. Scripture shows Israel's immorality was forgivable, but they could not enter His rest because they believed not. God can forgive the vilest sin to the uttermost, yet when people refuse to believe the gospel—that Christ died for them, paid their sin debt, and rose from the dead—their doom is sealed.
Les explains that although Israel committed vile immorality with the golden calf while Moses was up in the mount, God did not even mention that as what He was holding against them; He could forgive that kind of sin. Their problem was unbelief—they could not believe what He said. The passage says, "to whom swear He that they should not enter into His rest, but to them that believed not," and the last verse says they could not enter in because of unbelief. Les says nothing has changed: God can forgive to the uttermost any sin, the vilest of sin, except one—unbelief. When people refuse to believe that Christ died for them, paid their sin debt, and rose from the dead in power, and refuse that gospel, their doom is sealed. Then Les returns to Matthew 12.