What is the unforgivable sin?

‘And the teachers of the law…said, “He is possessed by Beelzebul! By the prince of demons, he is driving out demons.” So Jesus called them over to him and began to speak to them…If a kingdom is divided against itself, that kingdom cannot stand…In fact, no one can enter a strong man’s house without first tying him up. Then he can plunder the strong man’s house. Truly I tell you, people can be forgiven all their sins…but whoever blasphemes against the Holy Spirit will never be forgiven; they are guilty of an eternal sin.” He said this because they were saying, “He has an impure spirit.”’ – Mark 3:22—30
Unfortunately, the learned scribes of Jesus’ day maliciously labeled His ministry with the impudent irreverent brand of being in alliance with the devil; claiming that His expelling of devils was not because of His authority over them but merely by the consent of the demons.
This argument Jesus whitewashed by showing the absurdity of believing that a standing kingdom could possibly be at war with itself; if there was a pervasiveness of satanic activities among them, which was quite evident, then Satan’s kingdom is not at war with itself but at operative synergy. Secondly, Jesus silenced these religious teachers as He explained Satan’s subtlety; Satan will never voluntarily quit his possession. To dispossess the devil of the souls of men, he has to be disarmed, hemmed in and then ‘robbed’. He will not in any way let off easily; he is the valiant (strong man) and won’t go down without a fight!
The truth is the ministry of Christ is on opposite course to the kingdom of Satan, for this reason, no concerted effort can exist between them. Then in verse 28, Jesus identified an “eternal sin” that cannot be forgiven—blasphemy “against the Holy Spirit”. What is blasphemy against the Holy Spirit? It has been described as many things including speaking in tongues at will, lying in church, holding church leaders accountable, and so on. As we have seen the pretext to vs. 28, it is obvious that whatever blasphemy “against the Holy Spirit” is, it is not any of these things I have mentioned.
To begin with, the comment from verse 30—“for they were saying” tells of Jesus’ response to the scribes, showing that the blasphemy Jesus had in mind is somewhat verbal. The scribes seem to be erring with words; with testimonials against our Lord. Again, this same comment also tells of the unforgivable blasphemy being a persistent sin; an ongoing action, a resolute and deliberate decision. These religious teachers spoke against Jesus not just once; they were so cynical against Him that they unrelentingly associated Him with Satan, thus denying and rejecting His divinity!
Such defiance and resolute is really significant because it came from those with scriptural knowledge. Christ must have expected that with their knowledge of the Hebrew Scriptures (Old Testament), they would be able to identify God’s work (John 3:10) and His Messiah. Hence, the blasphemy of the Spirit does not arise from mere ignorance (as could be the case in Mark 6:1-6). When people know the Scriptures well or have heard and understood the gospel message, and yet not only fail to recognize Jesus as Messiah but also openly reject Him, they are standing on the dangerous ground; they are guilty of an eternal sin.
Blasphemy against the Spirit, then, is not lying in church, the random bad thought, or an episode of momentarily ‘anger’ against God. These things are no doubt evil, but they are definitely not the insistent, deliberate rejection of the Lord’s work and His Messiah as it is manifest in the willful attribution of God’s redemptive work (in and through His Son) to Satan himself and not to the Spirit of God. Such blasphemy is unforgivable not because God is reluctant to forgive but because such sin has fully (and to the very end) rejected God’s forgiveness and grace (offered only in the Person and Ministry of His Son Jesus). Anyone who does not love the Lord Jesus is accursed even until the day of the Lord but the grace of God is upon the Church (1 Cor. 16:22-23)
Finally, seeing Jesus as an agent of darkness is not mere defamation, it is calling the Holy Spirit of God a spirit of darkness; rebuffing the redemptive work of God, rejecting salvation through Christ Jesus and this is an eternal sin without forgiveness.