What does Hebrews 10:29 mean?

Under the old covenant, those who intentionally rebelled against God's law were subject to the death penalty (Numbers 15:27–31). So far, the book of Hebrews has explained in great detail how the new covenant is superior to the old covenant. It is composed of a greater promise (Hebrews 8:6), performed by a greater priest (Hebrews 7:28), enacted in greater places (Hebrews 9:24), and uses a greater sacrifice (Hebrews 9:12–14). Since the new covenant is far superior to the old, it follows logically that those who intentionally violate the new covenant would be subject to significantly more dire consequences.

Earlier, the writer spoke of those who "go on sinning deliberately after receiving the knowledge of the truth" (Hebrews 10:26). Some interpret this as a reference to people who have clear, obvious understanding of the gospel, but choose to reject it anyway. The more likely interpretation, especially in light of this verse, is that this is a warning to Christian believers, similar to the ones given in chapters 3 (Hebrews 3:12–19) and 6 (Hebrews 6:1–8). This verse specifically refers to the offending party as one who "was sanctified" under the blood of the new covenant. The context of the New Testament, and the book of Hebrews, indicates that salvation cannot be lost (John 10:28–29). These are not Christians falling from grace into damnation; rather, these are believers failing to obey and suffering the consequences.

Chapter 6 warned about the consequences of falling into doubt and disbelief. This included a reference to fire, as a symbol of God's judgment (Hebrews 6:8). Verse 27 gave that theme an even more dramatic description. God brought earthly judgment against His chosen people in the Old Testament. Their suffering was all the more severe since they, of all people on earth, knew better and should have obeyed. Christians who fall into doubt, disbelief, or disobedience, in the same sense, are subject to "much worse punishment" from God, since they ought to know better.