The Bible tells us that "blaspheming the Holy Spirit" is an unforgivable sin:
Matthew 12: 30. Whoever is not with me is against me, and whoever did not gather with me, scatters. 31. Because of this I say to you that all sins and blasphemies will be forgiven to the sons of men, but blasphemy that is against the Spirit will not be forgiven to the sons of men. 32. And anyone who says a word against the Son of man, he will be forgiven, But anyone who will speak against the Ruach haKodesh, he will not be forgiven - not in this world or in the world that is to come. (Aramaic English New Testament)
YHWH is the One True Universal Authority Who chose to be represented by His Mashiyach. But man wars against Yeshua and one another to establish his own authority. Dividing and scattering, fallen man inadvertently follows the pattern of haSatan, the Adversary.
If a person committed one of the capital offenses that are mentioned in Torah, but confessed and pleaded for forgiveness, there were ways to receive atonement. But those who blaspheme against the Spirit of YHWH have no way to make atonement for themselves, nor can they make an appeal in the name of Yeshua. See also Acts 5:1-11 and Hebrews 10:28-30.
Put another way, blasphemy against the Holy Spirit is an eternal sin, because if one rejects the evidence provided by the Holy Spirit and ascribes it to God's enemy haSatan, he rejects the only evidence upon which faith can be based.
Matthew 13: 32. And anyone who says a word against the Son of man, he will be forgiven, But anyone who will speak against the Ruach haKodesh, he will not be forgiven – not in this world or in the world that is to come.
Mark 3: 28. Truly I say to you, that all sins and blasphemies that the sons of man will blaspheme will be forgiven them. 29. But he who blasphemes against the Ruach haKodesh has no forgiveness ever, rather he is guilty before the eternal judgment. 30. This is because they were saying that an unclean spirit is in him.
Acts 5: 1. And a certain man whose name was Khanan-Yah, with his wife whose name was Shapeera, sold his field. 2. And he took from its sale price and hid it while his wife knew of it. And he brought some of the money and placed it before the feet of the Shlichim. 3. And Shimon said to him, “Khanan-Yah, why is it that the Adversary has filled your heart thus that you should lie in (the presence of) the Ruach haKodesh, and that you should hide that sum of the money of the proceeds of the field? 4. Was it not yours before it was sold? And after it was sold, again you were in control over its proceeds. Why have you placed in your heart to do this spectacle? You have not lied (in the presence of) men, but in (the presence of) Elohim.” 5. And after Khanan-Yah heard these words, he fell down and died. And there was a great fear among all those who heard. 6. And those who were young among them rose up and they gathered him together and took him outside and buried him. 7. And after three hours had passed, his wife also entered while not knowing what happened. 9. Shimon said to her, “because you have deliberated to tempt the Spirit of Master YHWH, behold the feet of the grave diggers of your husband (are) at the door, and they will take you out.” 10. And at that moment, she fell before their feet and died. And those young men entered and found her dead, and they gathered (her) up, took her away, and buried her next to her husband. 11. And there was a great fear in all the assembly, and in all those who heard.
Hebrews 10: 28. For if he who transgressed the Torah of Moshe, died without mercies at the mouth of two or three witnesses; 29. how much more, do you think, will he receive capital punishment who has trodden upon the Son of Elohim and has accounted the blood of his covenant by which he is sanctified, as the blood of all men and has treated the Spirit of grace in an insulting manner?
Acts 5: 32 And we are witnesses of these words and the Ruach haKodesh; He Who Elohim gives to those who obey Him.
The bottom line is: It is "defiant irreverence" - a term that can be applied to such sins as cursing Yahweh or willfully degrading things relating to Him, or attributing some evil to Him, or denying Him some good that should be attributed to Him. For instance, in Matthew 12:31-32, the Pharisees, having witnessed irrefutable proof that Yeshua was working miracles in the power of the Ruach haKodesh (Holy Spirit), claimed instead that He was possessed by the demon "Beelzebub" (Matthew 12:24).