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 ongoing, willful, habitual 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). *Hermeneutics.
If the word 'sanctified' in Hebrews 10:29 is used to describe saved people who lost their salvation as eternal IN-securists teach, then we have a contradiction because the writer of Hebrews in verse 10 said "sanctified through the offering of the body of Jesus Christ once for all" (Hebrews 10:10) and in verse 14, we read, "perfected for all time those who are sanctified." (Hebrews 10:14) I've heard eternal IN-securists try to get around the truth here by either saying forever doesn't really mean forever or "once for all and for all time" ONLY applies to "the offering of the body of Jesus Christ," but not to the person who is sanctified. Yet in Hebrews 10:10, we clearly read ..WE have been sanctified through the offering of the body of Jesus Christ once for all. In Hebrews 10:14, we read - For by one offering He has perfected for all time THOSE who are sanctified. To go from sanctified back to un-sanctified would be in contradiction here.
*NOWHERE in the context does it specifically say the person who "trampled under foot the Son of God, and has regarded as unclean the blood of the covenant" was "saved" and/or "lost their salvation." 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.
Strong's Concordance
hagiazó: to make holy, consecrate, sanctify
Original Word: ἁγιάζω
Part of Speech: Verb
Transliteration: hagiazó
Phonetic Spelling: (hag-ee-ad'-zo)
Definition: to make holy, consecrate, sanctify
Usage: I make holy, treat as holy, set apart as holy, sanctify, hallow, purify.
*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 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 seventh day was saved (Genesis 2:3), the tabernacle was saved (Exodus 29:43), Moses saved the people after coming down off the mountain (Exodus 19:14), the priests and the Levites saved themselves (1 Chronicles 15:14), the Father saved the Son (John 10:36), the Son saved Himself (John 17:19) 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 make believers/nominal Christians, 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 professing believer in the Hebrew Christian community of believers, but later renounces 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 Christian community of believers was only superficial and that he was not a genuine believer.