I've said this several times elsewhere, but it bears repeating.
Hebrews 10 is discussing the Jewish sacrificial system, that's why it's written to Hebrews.
What were Hebrews? Jews.
Now these Jews were considering going back to the old sacrificial ways.
And what do we know about the old sacrifices? They pointed to Christ.
And since they pointed to Christ, when He came, they were no longer effective in absolving sin.
So if you TURN AWAY from Christ there is no sacrifice remaining for you. Because that would mean He would need to be sacrificed again. His sacrifice was ONE time and it was enough for ALL time.
We know that because Hebrews 10:14 tells us this:
For by one sacrifice he has made perfect forever those who are being made holy.
One of the biggest issues with understanding Scripture is when we read it with an orphan mindset. If we are reading it with the lens of what we must do FOR God, we are almost always missing it.
Because we now live WITH God. He is one with us, His name is Emmanuel. His Spirit is our teacher. And He is our Lord. And God is our Father. Please notice the following attributes God defines Himself as: Father, Son, Teacher, Elder Brother, Shepherd, King... every single one of these denotes RELATIONSHIP.
So what is sin in context of relationship with Christ? I could go deeper into this, but I'll let Christ speak for Himself:
John 16:8And having come, He will convict the world concerning sin, and concerning righteousness, and concerning judgment: 9concerning sin indeed, because they do not believe in Me; 10and concerning righteousness, because I go away to the Father and you behold Me no more; 11and concerning judgment, because the ruler of this world has been judged.
Matt 25:40 And the King will reply, 'Truly I tell you, whatever you did for one of the least of these brothers of Mine, you did for Me.'
Bless you