The sin spoken of in Heb 10:26 is not sin in general, but the specific sin of apostasy.
Here is the last part of an article I wrote on this. You can read the whole article here if Interested.
http://www.nlbchapel.org/heb6.htm"Another explanation which I believe is the correct one is that the writer of Hebrews is using a hypothetical situation to make a point. This is the only explanation that I am aware of the does not violate other Scripture. The writer of Hebrews is concerned that some are turning back to the Old Covenant ways so he sets out to prove how foolish this is in light of how much better the New Covenant is. Remember, the book of Hebrews is a contrast between the Old and New covenant. His argument would amount to something like this for Hebrews 6:4-6 (the verses at the top of this article).
If it were possible for someone who is truly saved (verses 4-5) to fall away and abandon the salvation he has already received by the sacrifice of the Lord Jesus (verse 6). It would be impossible for him to be renewed to repentance (salvation) because in order for him to be renewed he would have to crucify the Son of God again, and put him to an open shame which will never happen (verse 6). The point being that outside of the sacrifice of the Lord Jesus in the New Covenant there is no salvation.
I believe this explains Hebrews 10 as well.
Hebrews 10:26 “ For if we sin wilfully after that we have received the knowledge of the truth, there remaineth no more sacrifice for sins, 27 But a certain fearful looking for of judgment and fiery indignation, which shall devour the adversaries. 28 He that despised Moses' law died without mercy under two or three witnesses: 29 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?”
The argument is something like this. If it were possible for a true Christian to sin willfully after receiving the gospel truth by rejecting it and therefore losing their salvation, there would be no hope of him ever getting saved again. The only thing awaiting him is the fiery indignation of God seeing that he has trodden under foot the Lord Jesus Christ and counted His blood by which he was saved an unholy thing. The only way of salvation is through the blood of Jesus and if a true Christian COULD reject this then there is no hope for him, seeing he rejected the only thing that could save his soul.
The hypothetical is the only way I know of that allows these verses to fall in perfect harmony with the rest of Scriptures. I hope this may help some. May God open all of our eyes of understanding."
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