I understand the "unforgivable sin against the Holy Spirit" is Unforgiveness which causes a vicious cycle of refusing God's forgiveness grace and ability to redeem through Christ Jesus or Restorative Justice. This unforgiveness prevents wrongs from being addressed and corrected as forgiveness allows God's love and truth in that sets us free.
This happens when you allow yourself to have a "reprobate" mind. A good example of such a sin is when one comprehends/understands with premeditation that their actions are sinful.......and make a decision to do that which they know conflicts directly with the Holy Spirit of God.......you can lie to yourself but you can't lie to God. That which remains unrepentant.......cannot be forgiven, its a sin that leadeth unto death.
Acts 5:3-4. "But Peter said, "Ananias, Why has Satan filled your heart to lie to the Holy Spirit and to keep back for yourself part of the proceeds of your land? While it remained unsold, did it not remain your own? And after it was sold, was it not at your disposal? Why is it that you contrived this in your heart? You have not lied to man, but to God."
John writes of a sin.........not leading to death, and a sin leading to death (1 John 5:16-17). According to James a sin that leads to eternal death is a sin that is fulfilled completely (James 1:15) Such a sin is a sin that a person continues in, does not admit to ever engaging in such a sin to any person, and never repents of that sin.......it then becomes unforgiveable, that which is not confessed and reprented becomes a death sentence for the spirit.
Paul speaks of such people whom God has given up on..........casting them away because they with full knowledge of God's existence and authority gave themselves over to a reprobate mind, they in essence ignored all warnings and advise from the Holy Spirit of God that showed them that God existed by the things that He made/created, by ignorning that which is apparently clear and unambiguous, they have called God a liar for they do not respect God, might alone Fear Him. (Romans 1:28)
In Acts 5:3-4............Ananias basically called the Holy Spirit of God a liar.........that was an unforgivable sin because Ananias and his wife both became servant to their reprobate mind due to greed.
Jesus Himself gives another example of the "unforgivable sin", He called it "Blasphemy Against the Holy Spirit of God" (Mathew 12:31-32, Mark 3:28-30). Jesus was speaking of the scribes and Pharisees when they accused Him of casting out demons by the power of Satan (Matt. 12:24), Jesus refuted their conclusions and stated He cast out demons by the power and authority of the Holy Spirit of God. They had accused Jesus of having an unclean spirit (Mark 3:30), thus they were saying that the Holy Spirit of God was unclean.......or a lying God, that is Blasphemy.........an unforgivable sin because the one's that have called God a liar and do not fear His authority and power.....actually believe the lies they are promoting and will never change, because God has given up on ever changing their unholy attitudes. The very first step to etenral salvation begins with belief/faith.......without belief, no sin can be forgiven, one must have faith in the One that has taken our sin upon Himself......Christ Jesus. Void of this faith.....you are doomed to die in your sins.