Mark 3:28-30 Verily I say unto you, All sins shall be forgiven unto the sons of men, and blasphemies wherewith soever they shall blaspheme: But he that shall blaspheme against the Holy Ghost hath never forgiveness, but is in danger of eternal damnation: Because they said, He hath an unclean spirit.
The Holy Spirit’s mission on earth is to point people to Jesus Christ. All of the miracles and wonders that were done by Jesus on earth were done by The Spirit of God. Demonic powers were cast out of people by the power of The Spirit. All manner of sicknesses and diseases again were healed by the power of The Spirit. The lame could walk, the blind could see, the dead were raised. So many miraculous things were done by the power of The Spirit that John wrote that earth itself could not contain the books to record them all (John 21:25). The purpose of every single one of these miracles was meant to bring God glory and reveal that Jesus is The One and only Savior of the world.
John wrote in chapter 16:7-11, Nevertheless I tell you the truth; It is expedient for you that I go away: for if I go not away, the Comforter will not come unto you; but if I depart, I will send him unto you. And when he is come, he will reprove the world of sin, and of righteousness, and of judgment: Of sin, because they believe not on me; Of righteousness, because I go to my Father, and ye see me no more; Of judgment, because the prince of this world is judged.
The unpardonable sin is rejecting the truth about Jesus Christ. It is believing that all roads lead to God. It is believing that salvation is a reward that is given to those who do enough good works or adhere to particular rituals or sacraments. Salvation is only by grace through faith in Jesus Christ apart from any works (Ephesians 2:8-9). To reject that message is the blasphemy of The Holy Ghost and unpardonable.