The unforgivable sin of blasphemy against the Holy Spirit is an act of resistance which belittles the Holy Spirit so grievously that he withdraws forever with his convicting power so that we are never able to repent and be forgiven
If a person refuses to put the ways of mankind behind in favour of those of the Kingdom then repentance is impossible and the HS has no one to guide. It all goes back to why Jesus said He would sent the HS. It all relates to why He came in the first place, the Kingdom. The HS was only for those of the Kingdom.
Luke 4:43 And he said unto them, I must preach the kingdom of God to other cities also: for therefore am I sent.
John 14:16 And I will pray the Father, and he shall give you another Comforter, that he may abide with you for ever; 17 Even the Spirit of truth; whom the world cannot receive, because it seeth him not, neither knoweth him: but ye know him; for he dwelleth with you, and shall be in you.
John 14:26 But the Comforter, which is the Holy Ghost, whom the Father will send in my name, he shall teach you all things, and bring all things to your remembrance, whatsoever I have said unto you.
"Even the Spirit of truth; whom the world cannot receive, because it seeth him not" ... the world refuses to reject itself in favour of the Kingdom, it refuses the Kingdom
"whatsoever I have said unto you." ... what did He teach? The gospel of the Kingdom