Blasphemy against the Holy Spirit: unforgivable

“Truly, I say to you, all sins will be forgiven the children of man, and whatever blasphemies they utter, but whoever blasphemes against the Holy Spirit never has forgiveness, but is guilty of an eternal sin” — for they had said, “He has an unclean spirit.”   Mark 3:28-30

From reformedanswers.org:

It is true that blasphemy of the Holy Spirit can never be forgiven. Although the Old Testament Law does not state this explicitly as such, this subject falls under the broader prohibition against using the Lord’s name in vain (Exod. 20:7) and loving the Lord with all your heart, soul and might (Deut. 6:5). In any event, Jesus himself specifically stated that blasphemy of the Holy Spirit was the unforgivable sin, so no amount of rationalization about the specific wording of the Old Testament laws can rightly conclude that it is not a sin, and no amount of rationalizing about John’s general statement about forgiveness can exclude the specific exception mentioned by Jesus.

But judging whether or not you have blasphemed the Holy Spirit is not really your call. You are not your judge — Jesus is. You may feel that you have blasphemed the Holy Spirit, but that doesn’t mean you have. In the only examples of such blasphemy that we have in Scripture, the blasphemers attributed the clear works of the Holy Spirit (namely miracles) worked by Jesus himself to the devil. You didn’t see the Lord perform any miracles, which means that whatever you did is not nearly as clear as a case as the one we find in Scripture. Although you might have attributed to the devil works that you now believe were performed by the Holy Spirit, this is not proof that the Holy Spirit performed the works in question.

An easier way to evaluate this question is to look at your heart and the content of your faith. A believer can never commit the sin of blasphemy of the Holy Spirit — it is impossible for him to do. We know this because the Bible teaches that salvation can never be lost. But if a believer were to blaspheme the Holy Spirit, he or she would certainly be lost. Therefore, a believer cannot commit that sin. Moreover, an unbeliever who commits that sin can never come to faith. If you now believe the true gospel and love the Lord, then you have never blasphemed the Holy Spirit.

After all, it is the Holy Spirit himself who indwells believers and moves them to act and to will according to his purposes (Phil. 2:13). Can you imagine that the Holy Spirit would move anyone to blaspheme himself? Of course he wouldn’t. That would make his motives self-contradictory. It is his job to keep us secure in Christ, not to permit us to sin our way into irrevocable destruction.

If, however, you are not a true believer, if you do not believe the gospel of grace that the Bible teaches and trust in Jesus as your Savior, if you do not love the Lord, then you may well have committed blasphemy of the Holy Spirit. But even then I would encourage you to seek and pursue the Lord, believe on him and repent of your sins. If you can do that, then you were wrong about your previous estimation of your actions.

And also from this article:

Blasphemy in general is “speaking against.” “Speaking” is pretty self-evident, although by implication we should also include other forms of public discourse like writing. “Against,” however, is a fairly vague term. In this case, it refers to taking a position in opposition to that of which one speaks. To speak against God is to say things in opposition to God himself, not merely against what God says. So, bad theology is not equivalent blasphemy. Blasphemy refers instead to things like calling God a liar, or cursing or reviling him (e.g. Ex. 22:28). It may be direct, such as in saying evil things about God, or indirect, such as in making claims that cannot be true unless evil things are true about God. For instance, in Matthew 9 some people considered Jesus to have blasphemed when he assumed for himself the power to forgive sins. Jesus essentially claimed equality with God. And this would have been an indirect form of blasphemy had it not been true.