I think the two possible explanations for what "blasphemy of the Holy Spirit" means are 1) a lesser-to-greater rhetorical point distinguishing between Jesus and the God who sent Him to earth ("If you curse me, it's not as severe as if you curse the God who sent and empowers Me" — the context of Luke 12:10, one of the "blasphemy of the Holy Spirit" verses, is ambassadors on earth representing heaven), or 2) a synonym for rejecting the most clear evidence that He came from God, since the context in Matthew and Mark is Jesus' opponents saying He did miracles by the power of Satan. Jesus didn't say it's fine to insult Him or His disciples, but was addressing His opponents' willful denial that He was doing God's will. Also, Hebrews 12:25 expresses the heaven-earth/lesser-to-greater argument more clearly, which was the main Jewish kind of logic which rabbis such as Jesus were using: "See to it that you do not refuse Him who is speaking. For if those did not escape when they refused him [Moses] who warned them on earth, much less will we escape who turn away from Him [Jesus] who warns from heaven [and is equal with God]."
However, there are numerous ways Jesus' statement gets misinterpreted to the extent it ties some well-meaning people up in fear. Even if we don't have fear about this particular issue, my discussion of the subject is relevant to how we view God's forgiveness and what grace means. Following my writing of the earlier five parts, the person whom I had helped with the fear gained even more fear after he read a slew of counterarguments, which I have adapted into Part 6 here.
One harmful claim is that blasphemy against the Holy Spirit is the only sin that can't be "taken back." The way this is worded implies that we have to try to pretend we never did a sin, backtracking or hiding from it. Whoever uses this phraseology fails to see that God's forgiveness is not based on our backtracking ("Oops — I didn't mean to sin; please don't punish me!"), but because He knows we are incapable of pleasing Him in our own strength. Forgiveness is about giving up the right to punish another, which is foundational to reconciliation. God forgives and also reconciles. He doesn't just tolerate us when we come to Him in faith, but responds with warm favor. Sin holds us back, while God wants us to be healed. When we feel like God must hate us, it's because our sin holds us back from receiving His love. Nowhere in the New Testament, just places in the Old Testament, does the attitude come across that God hates someone forever and will never forgive them regardless of whether their hearts change.
In human terms, true forgiveness works this way: If I have committed adultery, I can't undo the pain I've caused. I can help bring about healing if the other people involved are willing to work through the pain with me and not hold a grudge (which is a form of punishment against me), but I can't "take it back." What's done is done. Once you've said a harsh word, sometimes the relationship is over, but that's only if your relationship is weak. Any strong relationship will be able to move past any mistakes. A relationship with God that is based on always saying the right thing in your prayers and never blaspheming the Holy Spirit cannot be considered much of a relationship, as if it's our responsibility to make sure God likes us. The only thing which would make us unforgivable is our resistance, not His own, for He wants to reconcile with everyone.
I could tell someone, "If you feel scared of committing the unforgivable sin (blaspheming the Holy Spirit), that's proof that you didn't commit it, because you wouldn't even care about pleasing God if you were truly as spiritually hopeless as Jesus' opponents" — but the counterargument says, "The demons believe also, but they can't be saved," quoting James 2:19. My resolution of this is that fear, in its proper role, helps us see what our sin does to us, but should only be a temporary stage. We should never get to the point of worrying we're like the demons who can't be saved. First John 4:18 is an amazing verse because it is one of the only places in the Bible which says too much fear of God is unhealthy. You don't see that message much in the Bible. It says, "There is no fear in love; but perfect love casts out fear, because fear involves punishment, and the one who fears is not perfected in love."
Another point the legalists make about "blasphemy of the Holy Spirit" is that the Pharisees were knowledgeable of the Old Testament but ignorant of Jesus' divinity, and that ignorance didn't excuse them from Jesus' accusation of an unforgivable sin, so you can still blaspheme the Holy Spirit even if you don't do it "on purpose." Ronald Reagan made a similar kind of comment that liberals "know so much that isn't so" (in his breakout speech "A Time for Choosing" in 1964): it's either knowledge that is untrue, or knowledge that is interpreted incorrectly. The data is wrong, or the interpretation is wrong. But this debate about whether ignorance excuses you from the unpardonable sin is an unhelpful tangent. Ignorance is not itself sin; it just leaves people susceptible to false thinking. The Pharisees actually believed the same Bible Jesus believed, but they ignored many salient parts, including about God's love for all of humanity in Isaiah and other psalmic and prophetical literature. Ignorance is dangerous, but it's not an essential part of interpreting "blasphemy of the Holy Spirit." It was sinful misinterpretation of the info they were given, not ignorance per se, which was the problem here.
If blasphemy against the Holy Spirit is defined as simply a lack of discernment, then everyone who is opposed to Biblical morality about sexuality and everyone who persecutes or criticizes Christians' piety is guilty of it. Saul of Tarsus wouldn't have become the Apostle Paul if lack of discernment about God was an unforgivable sin. He was blaspheming the Holy Spirit, too, by considering the early Christians evil, even if He didn't witness the early Christians' miracles prior to his conversion — which means it was a forgivable sin after he stopped doing it. It's usually too hard to determine whether ignorance is unintentional vs. willful/hard-hearted. Paul was hard-hearted, but he didn't know the true nature of Jesus. He probably would have said his ignorance was willful. God is willing to save anyone. The only limit is on whether they're willing to be saved!
Another issue in these counterarguments' wording is that they talk about how "easy" it is to unknowingly commit the sin, when the proper terminology is not "easy" or "hard" but "natural" or "unnatural". It's still hard to do the right thing even when God has transformed your nature, but it also becomes hard to do the wrong thing! "Easy" and "hard" are not the best descriptors of the struggle for holiness. This is the mystery of virtue: You sometimes are driven by a spiritual nature to do something you wouldn't naturally do, but you are naturally doing it, even while you feel the resistance. "Natural" vs. "unnatural" is a more helpful distinction than "easy" or "hard" for this "blasphemy of the Holy Spirit" issue. It's unnatural for someone who is rich and self-reliant to trust in God and have humility; it's natural for people to be blind to God's revelation and succumb to false beliefs. So don't we all blaspheme the Holy Spirit when we call evil good or good evil, because it is natural for us to see God the wrong way? This broader interpretation of "blasphemy of the Holy Spirit" is implied in that Jesus even put the statement in the context of good and evil fruit and careless speech in Matthew 12:32-37.
Another reason the statement about the unforgivable sin shouldn't be a cause for fear is that Jesus says nothing about whether the people who blaspheme the Holy Spirit are sorrowful or genuinely repentant. (Repentance means changing your course.) There is no distinction between superficial regret/sorrow and repentance made in His statement — which the legalists would say is proof that it applies to anyone who does the sin, but which I could argue in response is a reason not to make too much of the statement.
Fear and guilt are sometimes evidence you did commit a sin, but they are sometimes imposed on you from outside yourself, preying on your insecurity. They are only good if they serve as warning signs or lead you to repentance — but then, if you have repented, the sin has been forgiven, and the guilt is gone (even if healing takes a long time). But in an unhealthily literal interpretation of "blasphemy of the Holy Spirit," Jesus said it "can't" or "won't" be forgiven, so the legalists argue that if you don't feel fear, you may have false security about your salvation, and if you do feel fear, you really have no hope because it's an unforgivable sin, if you've indeed committed it. Sounds like a great way to live...
There is one other counterargument which relates to the controversy over whether people are false teachers. It goes something like this: "Cessationists, who believe speaking in tongues and prophecy were spiritual gifts that expired after the apostolic era, are blaspheming the Holy Spirit when they don't acknowledge His work as His work." This is a question of discernment based on data given to us, and need not be about willful denial and pride. Whoever applies "blasphemy of the Holy Spirit" to the cessationist-Charismatic controversy is using a very low tactic of threatening damnation to those who are suspicious of the Charismatic movement. What a nasty potshot. Charismatics are people who believe God still heavily works through prophecies and speaking in tongues, while cessationists believe the completion of the New Testament made that no longer necessary. However, many people who don't like the Charismatic movement's excesses are open to the possibility that God can work in direct ways to bring people to Himself. If He does it through dreams Muslims have of Jesus, as many stories say, He can still use any of those means — but where the Bible is present, it no longer becomes necessary. When the Holy Spirit is exalted in the Charismatic movement but the Son is ignored, it inevitably leads to chaos and strange heresies, because the Son no longer becomes the one whose words we're listening to. What makes something a heresy is its inconsistency with what Jesus and the apostles taught and what the church as a whole believes. It's not that the majority opinion is always right, but if you stand against the majority, you'd better have good reasons besides just your own impressions.
If there is a sin God can't/won't forgive, the cross is insufficient for sin and His love is limited — which is just not possible, because no one would willingly go through what He did if He intended it only to apply to "every sin except one" that doesn't even get properly defined in the text. There's nothing about it in the apostles' epistles (a nice tongue twister J ), and there's no reason to believe they worried about it. Jesus didn't make a distinction between "speaking" or "thinking" against the Holy Spirit, either. He in fact merged the two in the Sermon on the Mount, saying they go together in sin and righteousness, that it's possible to have a bad heart but good behavior but impossible to have a good heart but bad behavior (generally speaking). He demonstrated that it's not He who is unwilling to forgive, but others who are unwilling to be forgiven. John 6:37: "All that the Father gives Me will come to Me, and the one who comes to Me I will certainly not cast out."