ANSWER: It is to disbelive in the death and ressurection of our Lord Jesus Christ, to reject the offer of salvation from Jesus Christ, Denial of atonement of one's sins by the blood of Jesus Christ are, to refuse the atonement for one's sins by the blood of Jesus Christ, to refuse the acknowledgement of one's sins before God, to refuse repentance from sinful ways, remaining dead in trespasses. This will make such individual to remain in darkness, incuring the wrath of God upon them. It becomes unpardonable when such individual passes from earth to eternity in such condition. One has automatically blasphemed the Holy Spirit.
Many believe that it is blasphemy against the Holy Spirit, according to scriptures in Mark 3:28-29. The rejection of Jesus Christ indicates rejection of the Person of the Holy Spirit. A person cannot separate the two in this context. A person who never blasphemed the Holy Spirit but rejected Christ will never be accepted by God, Christ is now ultimate. Yet a person who received Jesus Christ has automatically received His spirit (The Holy Spirit).
After the death of Jesus Christ on the cross, all sins are pardonable. Because of the Power of the shed blood of Jesus Christ. Jesus spelt it out as He hung on the cross, the words "IT IS FINISHED" - All things had come to a conclusion by His death and resurrection. The price had been paid to cover all sins. All laws abolished. Jesus Christ arose above the Law and those who received Him rise above the law.