It seems many of a more lawful mind are determined to tell believers wilfull sin is somehow different and that we can be sinless.  not sure how that works.
			
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I would say if you no longer believe it's a sin anymore. So it's not falling into temptation it's just disagreeing that it's even bad or a temptation at all.
sin is rebellion.
Yes, sinning is missing the mark.
What is the definition of sin (missing the mark)?
Transgression of the law, 1John 3:4. Just ONE transgression/sin makes you guilty of ALL the law, James 2:10. Just ONE transgression is missing the mark.
Unrighteousness is sin, 1John 5:17.
Christians do not miss these marks because we're not under the jurisdiction of the law, for righteousness (Rom 8:2, Rom 10:4, Gal 3:25, Gal 5:18, 1Tim 1:90) thus we cannot be charged with sin/transgression of the law, Rom 8:33.
Christians cannot be charged with sin/unrighteousness, as our faith is counted for righteousness, Rom 4:5.
As long as we continue to believe on Jesus, then our life is hid with Christ in God (Col 3:3) and thus we are not missing the mark/sinning. This is not saying we are physically perfect in behavior/lifestyle. Instead it refers to our position in Christ, as believers. It's Christ's righteousness/holiness/sinlessness that covers us.
So how does one sin willfully?
They reject Christ, falling from grace.
And how does one fall from grace?
Gal 5:4
You have become estranged from Christ, you who attempt to be justified by law; you have fallen from grace.
Thus we see in Heb 10:26 that the willful sin refers to those who had truly accepted Jesus as Lord (as described in Heb 6:4-6), but then turned back to works of the law as proof of justification.
Heb 10:26
For if we sin willfully (turning back to the law thus making yourself a sinner again) after we have received the knowledge of the truth (John 3:16), there no longer remains a sacrifice for sins,
As confirmation of this willfully making yourself a sinner, see Gal 2:18.
For if I build again those things which I destroyed (righteousness by works of the law), I make myself a transgressor./Sinner.
Any true Christian who turns back to the law, determining righteousness by works, such make themselves a sinner, willfully.
Whatever the law says, it says to those under it, Rom 3:19.
It is those who were truly Christians (as described in Heb 6:4-6) who reject Christ by turning back to the law, or even mixing works of the law with grace, who are the willful sinners that Heb 10:26 speaks of.
It seems many of a more lawful mind are determined to tell believers wilfull sin is somehow different and that we can be sinless. not sure how that works.
Not wilfully is when it happened unplanned out of stupid it in the heat of the moment. Will fully is living together without being married and know it's wrong.
is either unforgivable by God?
No except when you Did That, Repented, were set free, mature believer and then go back to the world again. God Warned me with hell.
1 John 5:16
16 If any man see his brother sin a sin which is not unto death, he shall ask, and he shall give him life for them that sin not unto death. There is a sin unto death: I do not say that he shall pray for it.
Hebrews 6
4 For it is impossible for those who were once enlightened, and have tasted the heavenly gift, and have become partakers of the Holy Spirit, 5 and have tasted the good word of God and the powers of the age to come, 6 if they fall away, to renew them again to repentance, since they crucify again for themselves the Son of God, and put Him to an open shame.
      Hebrews 6
4 For it is impossible for those who were once enlightened, and have tasted the heavenly gift, and have become partakers of the Holy Spirit, 5 and have tasted the good word of God and the powers of the age to come, 6 if they fall away, to renew them again to repentance, since they crucify again for themselves the Son of God, and put Him to an open shame.

 TPOG 
...
 the creator of Galatians 5:22 - 23.