Matthew 12:22-32 (ESV)
“22 Then a demon-oppressed man who was blind and mute was brought to him, and he healed him, so that the man spoke and saw. 23 And all the people were amazed, and said, “Can this be the Son of David?” 24 But when the Pharisees heard it, they said, “It is only by Beelzebul, the prince of demons, that this man casts out demons.” 25 Knowing their thoughts, he said to them, “Every kingdom divided against itself is laid waste, and no city or house divided against itself will stand. 26 And if Satan casts out Satan, he is divided against himself. How then will his kingdom stand? 27 And if I cast out demons by Beelzebul, by whom do your sons cast them out? Therefore they will be your judges. 28 But if it is by the Spirit of God that I cast out demons, then the kingdom of God has come upon you. 29 Or how can someone enter a strong man’s house and plunder his goods, unless he first binds the strong man? Then indeed he may plunder his house. 30 Whoever is not with me is against me, and whoever does not gather with me scatters. 31 Therefore I tell you, every sin and blasphemy will be forgiven people, but the blasphemy against the Spirit will not be forgiven. 32 And whoever speaks a word against the Son of Man will be forgiven, but whoever speaks against the Holy Spirit will not be forgiven, either in this age or in the age to come.”
In Matthew 12:31, the Pharisees are faced with positive evidence that Jesus is who he says he is, the Messiah. But rather than believe, they ascribe the work of the Spirit to the devil.
The sin at the time in its practice was obviously that the Jewish pharisees were attributing what Christ was doing to Satan but its main principle is a rejection of Christ and His ministry, unbelief and or apostasy. To reject the Christ and claim His works and ministry were of Satan was a clear evidence of both unbelief and an unrepentant heart. That’s why it was considered unpardonable.
Who is the strong man being bound? Who is he being accused of being in league with?
Satan is the strong man bound hence why Christ says “26 And if Satan casts out Satan, he is divided against himself. How then will his kingdom stand?” Jesus knowing their thoughts refutes the notion that He is under the power of Satan by telling them how absurd the notion is that Satan is casting himself out of people. The context calls for the fact that Jesus was talking about the Jewish pharisee’s particular rejection of Christ work and in principle rejecting Christ the Savior as a whole.
Christ is binding the strong man,Satan, by the very fact that He is casting out demons and taking over his house.
Louis Berkhof said this of the unpardonable sin: “In those who have committed this sin we may therefore expect to find a pronounced hatred to God, a defiant attitude to Him and all that is divine, delight in ridiculing and slandering that which is holy, and absolute unconcern respecting the welfare of their soul and the future life.” In view of the fact that this sin is not followed by repentance, we may be reasonably sure that they who fear that they have committed it and worry about this, and who desire the prayers of others for them, have not committed it.” (Systematic Theology, pp. 253-254.)
I conclude with Dr. John Calvin:
“From all that has been said, we may conclude that those persons sin and blaspheme against the Holy Spirit, who maliciously turn to his dishonor the perfections of God, which have been revealed to him by the Spirit, in which His glory ought to be celebrated, and who, with Satan, their leader, are avowed enemies of the glory of God. We need not then wonder, if for such sacrilege there is no hope of pardon; for they must be desperate who turn the only medicine of salvation into a deadly venom. Some consider this to be too harsh, and betake themselves to the childish expedient, that it is said to be unpardonable, because the pardon of it is rare and difficult to be obtained. But the words of Christ are too precise to admit of so silly an evasion. It is excessively foolish to argue that God will be cruel if he never pardon a sin, the atrocity of which ought to excite in us astonishment and horror. 25Those who reason in that manner do not sufficiently consider what a monstrous crime it is, not only to profane intentionally the sacred name of God, but to spit in his face when he shines evidently before us. It shows equal ignorance to object, that it would be absurd if even repentance could not obtain pardon; for blasphemy against the Spirit is a token of reprobation, and hence it follows, that whoever have fallen into it, have been delivered over to a reprobate mind, (Romans 1:28.) As we maintain, that he who has been truly regenerated by the Spirit cannot possibly fall into so horrid a crime, so, on the other hand, we must believe that those who have fallen into it never rise again; nay, that in this manner God punishes contempt of his grace, by hardening the hearts of the reprobate, so that they never have any desire towards repentance.”
http://www.biblestudyguide.org/comment/calvin/comm_vol32/htm/xiv.htm
Urban Reformed Theology 