The difference in how GOD deals with sin is not found in the severity or lack thereof of the disvalure of the sin as all sin has an equal and ultimate disvalue in GOD's sight.
It is the relationship of the sinner to GOD that changes how the sin is dealt with. The so called smallest sin by an elect puts Christ on the cross. The so called smallest sin by a reprobate puts them in hell.
The Unforgivable Sin
From my Christian pov, the unforgivable sin was the Satanic fall, their free will decision to reject by faith, that is, an unproven hope, YHWH's claims (NOT proof) to be our creator GOD and to reject the gospel of salvation from sin as found only in the Son. It contained the ideas that HE was no better than the rest of us so HE must be a liar and as a liar, a false god...just listen to atheists and pagans about their opinion of Jehovah for confirmation.
As the first liar in all of creation, HE must therefore be the most evil person in existence so they repudiated HIM, rejecting HIM from having any influence in their lives at all. Putting their faith in this idea that HE was evil and driven by a psychotic megalomania made them eternally unfit to ever be HIS Bride so they were condemned to judgement on the spot, passed over for election to salvation due to their choice.
They can't be forgiven because
1. they made the decision to rebel against GOD's claims to Deity by their free will and a free will decision cannot be changed by anyone, even GOD, unless the person asks for it to be changed. All free will decisions must be sacrosanct and inviolable, apart from GOD's interference or it cannot be defined as free. This is not a rule but a matter of definition:
A cannot be not A at the same time; wet cannot be dry at the same time; free to choose an unchangeable decision can't therefore be changed by another and still be considered free.
2. Once they rebelled they became enslaved by the addictive power of evil which destroyed their ability to seek true repentance and save themselves by changing their minds about HIM. They were instantly and totally unable to repent of their evil and became even more committed to the belief that they were right to rebel against this upstart liar and false god. So great is their addiction to evil that even after they learned the truth of HIS divinity and power when they saw the creation of the physical universe with their own eyes as described in
Job 38:7, they could not change their minds because they loved their sin more than the truth as we learn in
Roman 1:20+.
This describes the Satanic fall that precipitated the war in heaven and had all sinners, elect and reprobate, flung into the earth.
They are not unforgivable because HE hates them and their sin so much because it is worse than other sins OR for no reason at all but because when HE proclaimed his gospel of salvation to every creature created in HIS image,
Colossians 1:23, calling for us to accept HIS claims or to reject HIM, HE promised us our choice would not be interfered with or changed or forced upon us without our consent, though there would be legal and natural consequences...warnings which some ignored as lies.
The choice to repudiate HIS help in saving them from the grip of sin while becoming unable to save themselves by changing their minds sealed their doom.
So the only people in hell will be the ones who put themselves outside of HIS loving grace and mercy by their own free will decision. EVERYONE who can be saved will be saved. HE wants no one to suffer eternal death,
1 Timothy 2:4, and since HE does all for HIS own pleasure but takes NO pleasure in the death of the wicked,
Ezekiel 33:11,
HE did NOT create them to merely go to hell!!
Everyone who is damned is a person who put themselves outside of HIS gospel salvation by their own free will decision to rebuke HIM as a liar and a false god, the unforgivable sin. Damnation is by their will, not by HIS!!! That is the meaning of a true free will.