Blasphemy of the Holy Spirit is not believing that Jesus is the Christ. And normally, those who commit the unforgivable sin aren't worried about committing it, because they don't believe anyway.
John 16:8-9 "And when he is come (speaking about the Holy Spirit), he will reprove the world of sin, and of righteousness, and of judgement: of sin, because they believe not on me; (talking about Christ)"
So here we see that the Holy Spirit comes to reprove the world of the sin of unbelief. When a person rejects that reproof, he is likened unto a fool.
Proverbs 15:10 "Correction is grievous unto him that forsaketh the way: and he that hateth reproof shall die."
Proverbs 15:31-32 "The ear that heareth reproof of life abideth among the wise. He that refuseth instruction despiseth his own soul: but he that heareth reproof getteth understanding."
So proverbs speaks expressly about those that reject reproof, both the reproof of wise men, but more so the reproof of God, and how rejecting it leads to death.
The reason why blasphemy of the Holy Spirit is the unforgivable sin is because we literally reject the witness it has of Jesus Christ, and essentially call God a liar.
Romans 8:16 "The Spirit itself beareth witness with our spirit, that we are the children of God."
1 John 4:2-3 "Hereby know ye the Spirit of God: Every spirit that confesseth that Jesus Christ is come in the flesh is of God: And every spirit that confesseth not that Jesus Christ is come in the flesh is not of God: and this is that spirit of antichrist, whereof ye have heard that it should come; and even now already is it in the world."
1 John 5:9-10 "If we receive the witness of men, the witness of God is greater: for this is the witness of God which he hath testified of his Son. He that believeth on the Son of God hath the witness in himself: he that believeth not God hath made him a liar; because he believeth not the record that God gave of his Son."
Blasphemy itself is not bearing reverence toward the Lord, or it is a way of insulting God. Blasphemy could also be the act of speaking against the truth, or rejecting the truth. Like when the Pharisees thought that Jesus was speaking blasphemy when he called himself God - they thought he was lying. Obviously Jesus was telling the truth, but they thought he was speaking against the truth.
Rejection of the witness of the Spirit is blasphemy, because it is the rejection of the truth. Therefore, it is the unforgivable sin because a person cannot be saved without believing on the Lord Jesus Christ (Romans 10:9).
If you believe that Jesus is the Christ, you are born of God (1 John 5:1). And if you are born of God, you haven't committed, nor could you ever commit the unforgivable sin.
