In Hebrews 10:26, to "sin willfully" in the Greek carries the idea of deliberate intention that is habitual, which stems from rejecting Christ deliberately. This is CONTINUOUS ACTION - A MATTER OF PRACTICE. Now we don't walk along our daily life and accidentally fall into a pit called sin. We exercise our will but, the use of the participle clearly shows a CONTINUOUS ACTION. The unrighteous practice sin - 1 Corinthians 6:9-10; Galatians 5:19-21 not the righteous, who are born of God - 1 Corinthians 6:11; 1 John 3:9.
The reference to "the blood of the covenant that sanctified him" in verse 29 seems to be referring to a Christian, but this overlooks the fact that the word translated "sanctified" (which is a term often applied to Christians; it is the verb form of the adjective "holy") really just means "set apart," and doesn't necessarily refer to salvation. In 1 Corinthians 7:14, Paul uses it to specifically refer to non-Christians who are "sanctified" or "made holy" by their believing spouse. (And by this Paul does not mean that they are saved).
A non-Christian can be "set apart" and from other non-Christians and sinful things without experiencing salvation as Paul clearly explained. So the word "sanctified" means to be "set apart." If the word "sanctified" simply meant saved, then you would have to say that the Sabbath was saved (Genesis 2:3), the tabernacle was saved (Exodus 29:43), the Lord was saved (Leviticus 10:3), the Father saved the Son (John 10:36) and many other things that just do not line up with Scripture.
In verse 39, the author sets up the contrast that makes it clear to me that he was referring to unbelievers, not saved people: But we are not of those who draw back to perdition, but of those who believe to the saving of the soul. Those who draw back to perdition do not believe to the saving of the soul and those who believe to the saving of the soul do not draw back to perdition.
So after considering the context, it seems most likely that "he was sanctified" should be understood in the sense of someone who had been "set apart" or identified as an active participant in the Christian community of believers, but who has subsequently committed apostasy by renouncing his identification with other believers, by rejecting the "knowledge of the truth" that he had received, and by repudiating the work and the person of Christ himself. Such a person’s apostasy is thus evidence that his identification with the Christian community was only superficial and that he was not a genuine believer.