Hebrews is a big letter. Losing our salvation is in respects to grieving the prodding of the Holy Spirit. Christ through the Holy Spirit dwells in our hearts.
In Ephesians 4:30, Paul said - And do not grieve the Holy Spirit of God, by whom you were
sealed unto/for the day of redemption. Nothing there about losing salvation.
The word of GOD through Christ dwells in our hearts and mouths. The law through Christ is in our hearts and minds. We are a new creature in Christ Jesus. Old things have passed away; behold all things have become new and are of GOD. For it is HE that works in us both to will and do His good pleasure.
Philippians 1:6 - being confident of this very thing, that
He who has begun a good work in you will complete it until the day of Jesus Christ.
Therefore....... if we sin wilfully after that we have received the knowledge of the truth, there remaineth no more sacrifice for sins, But a certain fearful looking for of judgment and fiery indignation, which shall devour the adversaries. He that despised Moses' law died without mercy under two or three witnesses: Of how much sorer punishment, suppose ye, shall he be thought worthy, who hath trodden under foot the Son of God, and hath counted the blood of the covenant, wherewith he was sanctified, an unholy thing, and hath done despite unto the Spirit of grace? For we know him that hath said, Vengeance belongeth unto me, I will recompense, saith the Lord. And again, The Lord shall judge his people.... But we are not of them who draw back unto perdition (destruction); but of them that believe to the saving of the soul.
(Heb 10:26-30,39)
In regards to 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 "on the surface" appears to be referring to a Christian, but this overlooks the fact that the word translated "sanctified" (which is the verb form of the adjective "holy") which 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 "set apart" by their believing spouse and by this Paul does not mean that they are saved. A non-Christian can be "set apart" from other non-Christians 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 do not line up with scripture.
In verse 39, the writer of Hebrews 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 a participant in the Hebrew Christian community of believers, but who has committed apostasy by renouncing his identification with other believers, by rejecting the "knowledge of the truth" that he had received and trampling under foot the work and the person of Christ himself. This gives evidence that his identification with the Hebrew community of Christians was superficial and that he himself was not a genuine believer.