One of the big questions in the book of Hebrews concerns the recipients and the purpose of the warning passages like chapter 10. Was he writing to true believers with the possibility that some were only professing or merely apparent believers? Or was he writing to genuine believers of the serious consequences of apostasy from the standpoint of God’s judgment in this life and the loss of rewards in the future kingdom? I personally favor the last viewpoint. I believe the author is referring to genuine believers as the “sanctified.” The following comment from The Bible Knowledge Commentary is a good summary of the passage and fits with my understanding of the passage as well.
10:26-27. The KJV translation here, “if we sin willfully,” is superior to NIV’s if we deliberately keep on sinning, as the words “keep on” overplay the Greek tense. As the context shows (cf. v. 23), the author was concerned here, as throughout the epistle, with the danger of defection from the faith. Most sin is deliberate,” but the writer was here influenced by the Old Testament’s teaching about sins of presumption (cf. Num. 15:29-31) which lay outside the sacrificial provisions of the Law. Apostasy from the faith would be such a “willful” act and for those who commit it no sacrifice for sins is left (cf. Heb. 10:18). If the efficacious sacrifice of Christ should be renounced, there remained no other available sacrifice which could shield an apostate from God’s judgment by raging fire. A Christian who abandons “the confidence [he] had at first” (3:14) puts himself on the side of God’s enemies and, as the writer had already said, is in effect “crucifying the Son of God all over again and subjecting Him to public disgrace” (6:6). Such reprehensible conduct can scarcely be worthy of anything but God’s flaming indignation and retribution. This, however, as stated earlier (cf. comments on 6:8), is not a reference to hell (cf. comments on 10:29).
10:28-29. Under the Old Covenant, if an Israelite spurned the Mosaic Law and at least two or three witnesses verified his actions, he was put to death. This being true, the author then argued from the lesser to the greater. If defiance of an inferior covenant could bring such retribution, what about defiance of the New Covenant which, as he had made clear, is far superior? The answer can only be that the punishment would be substantially greater in such a case.
In order to show that this is so, the writer then placed defection from the faith in the harshest possible light. An apostate from the New Covenant has trampled the Son of God underfoot and has treated as an unholy thing the blood of the covenant (cf. “blood of the eternal covenant,” 13:20) that sanctified him. The words “sanctified him” refer to true Christians. Already the writer to the Hebrews has described them as “made holy (Gr. sanctified¯) through the sacrifice of the body of Jesus Christ once for all” (10:10) and as “made perfect forever” through this sanctifying work (v. 14). Some seek to evade this conclusion by suggesting that Christ is the One referred to here as “sanctified” or that the person only claims to be sanctified. But these efforts are foreign to the writer’s thought and are so forced that they carry their own refutation. The author’s whole point lies in the seriousness of the act. To treat “the blood of the covenant” (which actually sanctifies believers) as though it were an “unholy” (koinon, “common”) thing and to renounce its efficacy, is to commit a sin so heinous as to dwarf the fatal infractions of the Old Covenant. To this, an apostate adds the offense of insulting the Spirit of grace who originally wooed him to faith in Christ. This kind of spiritual rebellion clearly calls for a much worse punishment than the capital penalty that was inflicted under the Mosaic setup.
But again the writer was not thinking of hell. Many forms of divine retribution can fall on a human life which are worse than immediate death. In fact, Jeremiah made just such a complaint about the punishment inflicted on Jerusalem (Lam. 4:6, 9). One might think also of King Saul, whose last days were burdened with such mental and emotional turmoil that death itself was a kind of release.
10:30-31. No one should regard such a warning as an idle threat. God Himself has claimed the right to take vengeance and to judge His people. In saying this, the author quoted twice from Deuteronomy (32:35-36), a chapter which most vividly evokes the picture of God’s people suffering His retributive judgments (cf. esp. Deut. 32:19-27). Those familiar with this text, as well as other descriptions of God’s wrath against “His people,” agree: it is a dreadful thing to fall into the hands of the living God.