This passage provides a principle for those Jewish people who were in the first century church that had rejected the truth of the coming judgment of all. If they were genuine believers their spiritual eternal salvation was assured but they would suffer bodily in some manner. If they were not saved then the significance of their willful sin would render them unsaved and Hell is their destination. Paul previously warned that, by failing to fellowship, apostasy may already have taken place because some refuse the assembling together. Because some have already stopped associating with other believers, a warning follows. If you purposefully sin by returning to Judaism and leave Christ they were guilty of a massive sin.
This is not the accidental sins all commit from time to time out of ignorance. This is a willful sin. For this there is no remedy. If they are spiritually unsaved there is only Hell because there is no other atonement for their sin than Jesus’ work on the cross. If they are saved then the penalty is physical death.
While the cross covers all sins for eternity, there is a sin here that the cross of Christ will not cover in this life. This is a sin they continue to commit even though they know it is wrong. In this particular context, it refers to the Jewish people in the church going back to Judaism and remaining there. This action is actually a total denial of all their former beliefs and actions that were good and right. The major offence this sin has is the repudiation of their previous confession that Jesus is the Christ or Messiah.