THE UNPARDONABLE SIN
People permanently rejected by God (those damned), all have one thing in common.
THEY HAVE ALL SEEN AND EXPERIENCED THE POWER OF GOD, AND HAVE REJECTED THIS.
That is what it means to blaspheme against the Holy Spirit.
Let me give you a comprehensive list of all those the Bible says are damned forever.
Judas, Son of Perdition, saw Christ's miracles and rejected these.
The Pharisees, same reason. And it was a knowing rejection. They knew these miracles were from above, not below.
King Saul, chosen of God, empowered by the Holy Spirit to even prophesy, yet let his relationship slip, and ended up going to a witch.
The slave generation of Israelites who came out of Egypt, saw God's miracles, yet built a golden calf and ascribed their deliverance to this idol. God swore in His wrath that they would not enter into His eternal rest (Heb 3:11).
Adam - who walked and talked with God, then disobeyed, and never sought reconciliation. There is nothing more God can do which would persuade Adam to love and obey Him.
Let's have a look at Matt 13's blasphemy against the Holy Spirit.
31 Wherefore I say unto you, All manner of sin and blasphemy shall be forgiven unto men: but the blasphemy against the Holy Ghost shall not be forgiven unto men.
32 And whosoever speaketh a word against the Son of man, it shall be forgiven him: but whosoever speaketh against the Holy Ghost, it shall not be forgiven him, neither in this world, neither in the world to come.
This is spoken to the Pharisees who had just witnesses Christ's miracles. They were faced with two possibilities only. Either these paranormal events were from God or from the Devil. They wilfully chose to ascribe them to Satan, against even what their better judgment was telling them. They did not want to lose position and power, and they were not humble enough to submit to Christ's authority. They had everything to lose by following Christ. And they knew this.
Nicodemus spoke for all of the Scribes and Pharisees when he said…
John 3:2
The same came to Jesus by night, and said unto him, Rabbi, we know that thou art a teacher come from God: for no man can do these miracles that thou doest, except God be with him.
This ties in with Heb 6:4-6
4 For it is impossible for those who were once enlightened, and have tasted of the heavenly gift, and were made partakers of the Holy Ghost,
5 And have tasted the good word of God, and the powers of the world to come,
6 If they shall fall away, to renew them again unto repentance; seeing they crucify to themselves the Son of God afresh, and put him to an open shame.
It is possible for non-believers to be "once-enlightened". The Pharisees of Christ's day, are a case in point. They saw the miracles, and in this way "tasted of the heavenly gift". And they fell away, meaning deliberately and knowingly rejected Messiah.
They cannot be restored, ever, to favour with God.
When Christ, in Matt 23, seven times pronounced "Woe unto you, scribes and Pharisees, hypocrites!" He was passing final judgment upon them that they were forever damned.