It is blasphemy against the Holy Spirit (Matthew 12:31; Mark 3:29; Luke 12:10).
“The sin against the Holy Spirit has to consist in a conscious, deliberate, intentional blasphemy of the—clearly recognized yet hatefully misattributed to the devil—revelation of God’s grace in Christ by the Holy Spirit.
The blasphemy against the Holy Spirit, therefore, does not simply consist in unbelief, nor in resisting and grieving the Holy Spirit in general, nor in denying the personality or deity of the Holy Spirit, nor in sinning against better knowledge and to the very end without qualification. Nor is it a sin solely against the law, but also a sin specifically against the gospel, and that against the gospel in its clearest manifestation. There is much, therefore, that precedes it: objectively, a revelation of God’s grace in Christ, the nearness of his kingdom, a powerful working of the Holy Spirit; and subjectively, an illumination and conviction of the mind so intense and powerful that one cannot deny the truth of God and has to acknowledge it as being divine.
In this case it does not consist in doubt toward or the denial of that truth, but in a denial that contradicts the conviction of the mind, the illumination of the conscience, and the intuitions of the heart. It then consists in a conscious and deliberate attribution of what has been clearly perceived as God’s work to the influence and activity of Satan, that is, in a deliberate blaspheming of the Holy Spirit, a defiant declaration that the Holy Spirit is the spirit from the abyss, that the truth is a lie, that Christ is Satan himself.
Its motivation, then, is conscious and intentional hatred against God and what is recognized as divine; its essence is sin in its ultimate manifestation, the complete and consummate revolution, putting God in the place of Satan and Satan in the place of God. Its character is no longer human but demonic…
God’s grace is not too little and too powerless to pardon it; but also in the realm of sin, there are laws and ordinances that God has implanted in it and maintains. And in connection with this sin, that law consists in the fact that it rules out all remorse, scorches the conscience shut, definitively hardens the sinner, and in this way makes his sins unpardonable.”
-Herman Bavinck, Reformed Dogmatics: Sin and Salvation in Christ (vol. 3), pg. 155-6.