In short, it would be when a person decisively and finally rejects the testimony of the Holy Spirit about the person & work of Christ. In Jesus’ time, this testimony came via His own words and miracles – which were the work of the Spirit being manifested through Him. Today, this testimony would come through the written Scriptures in association with that “still small voice” of the Spirit speaking to the heart & conscience of individuals as He enlightens their understanding of the gospel and simultaneously convinces them of its truth. This is not something that happens at all times, but only at specific and limited times within each individual’s life. At some point, the individual rejects this testimony to the point that the Holy Spirit ceases to enlighten and convict and leaves the sinner in his condemned state, and thus there is no more hope of his getting saved. (Salvation can only occur when God is “drawing” the sinner to Himself in this fashion [John 6:44], so if the Spirit stops drawing, he is forever lost). As 1 John 5:9-10 says, it is a form of blasphemy because rejecting the testimony of the Holy Spirit is tantamount to calling God a liar. This would be the same sin spoken of in Hebrews as “sinning willfully after receiving a knowledge of the truth” (Heb. 10:26) and “falling away” after one has been once-for-all enlightened by the Holy Spirit (Heb. 6:4-6).

In summary, it is basically the sin of refusing to get saved when the opportunity of salvation is there. Thus, any sin can be forgiven with the one obvious exception that one cannot be pardoned if he steadfastly refuses to accept God’s gracious pardon. God does not forgive people who do not wish to be forgiven (at least, they do not wish to be forgiven on God’s terms, which involve an attitude of contrition & repentance and also a willingness to stake our forgiveness solely on the death & resurrection of the divine Christ as opposed to any merit of our own). Of course, not all theologians would agree with my answer, but I commend it to you as the best synthesis of the Biblical data.

While I can't comment on the theology, I can add a note that may prove helpful in this and other like instances.

Blasphemy in the Jewish order of things often includes something we hear little of in Christian circles. It includes doing a work of power, like a miracle or interpretation of dreams etc without attributing the source of the power or interpretation to the Giver, God. Recall various examples of this from Joseph, Moses, Daniel etc. On one signal occasion Moses failed to do this and lost his right to enter the promised land. Instead of referring to God, he says: "Hear now you rebels, shall I bring water to you from this rock?"

When you apply this to Jesus' many sayings, none of which was preceded by a mere prophet's "Thus says the Lord," and ending with an answer to the High Priest's adjuration : "I adjure you by the living God that you tell us you are the Christ, the Son of God." (Matt 26) "I am." Jesus was convicted in that court of blasphemy, and delivered up to Pilate on that charge.

Leviticus 5:1 is obviously induced tacitly here, as a well-known jurisprudential injunction.

The Holy Spirit is an active person in the plan of redemption. It is His role to convict the world of sin, righteousness and judgement [Jn 16:8]. This activity involves impressing upon the heart of the sinner to repent and be converted and be made (at-one-ment) with God through Christ. As man's hardened heart constantly refuses to yield to such impressions and convictions of the Spirit, he inevitably denies (literally blasphemes the Spirit). Whereas it is God's desire to save all mankind, the consistent denial of the ministeration of the Holy Spirit (which constitute the blasphemy of the Holy Spirit [Mt 12:31,32] leads to death (eternal alienation).

I think it likely that Jesus was really talking about a kind of hard-hearted and habitual sinning from which there is no return, not because God won't forgive, but because the person will never admit his wrong, or desire to rectify his wrong. The point was lost and the evangelists (mis)understand it as some kind of hard barrier against forgiveness.

I'll submit my opinion, please take it as such.

Jesus' answer to the High Priest tacitly asserted he was divine: not specifically Son of God as we now use that term, since they were unfamiliar with Father, Son, and Holy Spirit.

They were probably far more ready to receive the term "Holy Spirit" as indicating deity than that God had a Son, the second person of the Godhead. While Jesus never taught that doctrinally, his every word and doing proclaimed it for their confession, had they received it.

Jesus repeatedly states that he was speaking the direct words of God, or of the Father, another term they were unfamiliar with...Jesus being the first man on earth ever to look up and say "Father" instead of God or Shaddai or other terms in common parlance with the Jewish community.

Thus by telling them it was blasphemy to reject the Holy Spirit's witness to Jesus' words and works, they were guilty of blasphemy. The Holy Spirit is God, just as Jesus the Son of God, and just as the Father.

Jesus' doings and teachings asserted that he was the man whom Moses had led them to expect would succeed himself (see Deut 16 and Acts 7 especially verse 37 and verse 51). The leaders knew, tho they would not publicly discuss in the ears of the common people, that the coming messiah was more than a man, in fact, divine. They sidestepped Jesus' assertion of this fact in Matt 22:41-45 when he asked why, if the coming one were merely a son of David, why would David address him as Lord.

But the High Priest knew and stated plainly in the ears of all that he knew the Coming One was "the son of the blessed." Matt 26:63, thus letting slip the fact that made the judging High Priest guilty of blasphemy.

I'm quite open to your comments on this.

If I may add my own opinion, which is not to take issue with anyone else's.

As with any scripture, context is important, especially given the word “therefore” that leads into the passage about blasphemy against the Holy Spirit. The previous verses are essentially about the manifestation of the Kingdom of God and implications of Christ’s power and authority being from the Spirit of God and following that, whether people in their response are working with or against Christ. “If it is by the Spirit of God that I drive out demons, then the Kingdom of God has come upon you”, then “he who is not with me is against me”. So we are not just dealing with a failure of discernment, but a heart attitude that opposes the King and the establishment of the Kingdom of God. The verses that follow on speak of the nature of the heart and that only good trees bear good fruit, while those who are evil produce evil fruit. This implies a heart that is rooted in evil and willing gives itself over to it and not only resisting but utterly opposing the work of the Spirit.

Blasphemy against the Holy Spirit is at the extreme end of resisting the Holy Spirit, but it is not our job to determine where that point is or who has crossed the line. Our focus is more usefully put on how we take heed of the warnings personally and then help others to do so. The fear of God is a powerful aspect in convicting us of how any resistance to the Holy Spirit’s work affects God and us, (also is said that sin makes us stupid). We are accountable for the light that we are given by the Spirit, but the problem comes when our agendas take over from God’s e.g. how the Pharisees resisted the Spirit, outlined in Stephen’s speech in Acts 7:1.

At the end of the day we cannot be forgiven or cleansed from any sin unless we are convinced of it by the Spirits illumination of it and then confess/repent from them. So if we are ultimately opposed to the Spirit’s work this is not going to happen. All the same God knows those who are His, and will continue to work in them until the return of Christ, so really no one who is worried about committing the unforgivable sin has actually done it.

There are other related matters that the Bible clearly warns against, including false prophets, which once again compares to being a bad tree producing bad fruit and those who claim to have done great things in Christ’s name but for the wrong agenda out of a lawless and rebellious heart (Matt 7:15-23). Also with the sheep and goats judgement in Matt 25 the goats are told to depart because they never had never knew Jesus intimately, so ultimately it’s our relationship with him that matters.

Here's how I see the internal dynamic. Human freedom can be in an enslaved condition (original sin). Personal sin ratifies that slavery by refusing to heed the least light of conscience stirred by the Holy Spirit until the point comes that one no longer can tell the difference between good (eg Jesus but also other realities) and evil (Beelzebub, continuance in sin).

How can always available and effective divine forgiveness in Christ become powerful in someone unless he or she repents, in however preliminary and incomplete a conversion?

But how can someone repent when unwilling to walk out the door in our freedom that the Holy Spirit opens for us by drawing us to even the least glimmer of love for God and neighbor? If we refuse the opening door we refuse the Spirit, substituting our sinfulness for the Spirit, i.e blasphemy.

It seems to me from a careful study of the scriptures, as well as from what can be learned from more worldly sources, after all, scriptures themselves state that the heavens and the earth proclaim the glory of God, so worldly sources are legitimate, that blasphemy against the Holy Spirit must be a very serious crime. To claim that it is to reject Jesus seems possibly misleading, for is the person rejecting Jesus or rejecting the presentation, which might well include the person doing the presenting. Likewise, from Near Death Studies it seems that devote Christians and devote non-Christians have very similar experiences and subsequent responses, including rejecting formal religions including Christianity. Now based more on personal experience, and consistent with Hebrews 6. I would suggest that the only way commit blasphemy against the Holy Spirit is to have clear personal knowledge of what God offers, not just verbal but experiential, and then reject it. If having tasted the fullness of what God offers, one states, I can do better, and rejects it, there is no coming back.

Discussion of this question would most appropriately begin with reference to Mark 3:29.

The hardness of heart, not forgiving and rejecting forgiveness, stubbon proud till the end of life... are effectively some of the "sins against the Holy Spirit", because Spirit is love and mercy, and this are sins against God's love and mercy. You all have listed some of the sins against the Holy Spirit and their basis in the Holy Bible. Blasphemy is to use the Holy Name of God for (mere) human purposes, that is, in vain: for instance, when someone attibutes to humans what can be done only by God, when men use the divine gifts for human glory, disrespectful behaviour with God, etc. It's one of the most serious sins listed in the Bible. So, we can say that there are very serious sins against the Holy Spirit, which are the sins against the mercyful love of God for mankind. However, moving from exegesis to other theological fields, I will suggest to better precise the concept of "eternal sin". For instance, many Roman Catholics theologians state that there is no "eternal sin", because "eternity" is constant and complete possession of all good, something that can be identified with God. We can share eternity in the measure we share God's life. So, strictly speaking, for them "eternal sin" is somehow contradictory. They will use "perpetual sin", or "sin that can not be forgiven now and forever" (that is man's definitive rejection of God's salvation that comes through the gift of the Holy Spirit). Maybe this approach can be helpful to better explain men's proud and the closeness of men's heart in front of God's mercy, wich has strong biblical roots.