Jesus stated: "Assuredly, I say to you, all sins will be forgiven the sons of men, and whatever blasphemies they may utter; but he who blasphemes against the Holy Spirit never has forgiveness, but is subject to eternal condemnation" (Mark 3:28-29 NKJV). The answer to the question therefore has two parts:
1) What does it mean to blaspheme the Holy Spirit, and,
2) Why is there no forgiveness of this action?
These are important as some have worried hard about whether they have blasphemed the Holy Spirit or not, therefore, have had a troubled heart over their salvation.
There are two examples to answer this question.
In Leviticus chapter 24, Moses tells of a woman's son who blasphemed the name of Jehovah (Yewah). This man cursed God while he was fighting an Israelite man. Moses was asked to judge the case. It is interesting to note that this man's father was Egyptian but his mother was an Israelite; in latter days it was held a person required a Jewish grandmother to be Jewish, but in early Israelite history, people with non-Israelite mothers were considered Israelite. There appears to have been no precedent for this type of case, so they put the man into custody and Moses inquired of the Lord. The result was the Lord demanded the man be put to death by those who had heard him blaspheme God. What was said was not recorded, although history records that Egyptians cursed their idols when they appear not to listen. So perhaps this half Egyptian cursed the Lord God Most High in the same manner. From this arose a law:
"Whoever curses his God shall bear his sin. And whoever blasphemes the name of the LORD shall surely be put to death. All the congregation shall certainly stone him, the stranger as well as him who is born in the land. When he blasphemes the name of the LORD, he shall be put to death." (Leviticus 24:15a-16)
The second example occurs when the Pharisees and rulers reject Jesus as Messiah. In Matthew chapter 12:12 we see a very salient incident. Jesus healed a man who was both blind and mute (dumb) and who was demon-possessed. This was most unusual. To demand a demon come out of a person, one first had to ask his name, and the demon, having the power to take over the person's body, in particular the tongue, would answer. An example is given in Luke 8:30, where the name of the demon was Legion. In the Matthew 12 case, the man was blind and mute therefore the demon had no method of giving his name. However, Jewish tradition taught that the Messiah would be able to cast out demons even if the name was not known. The fact that demons in mute or dumb people were different to deal with explains the issue found in Mark 9:17 – 29 when Jesus's disciples could not heal a certain man's of demon-possession. When asked why Jesus explains that only by prayer and fasting could this type of demon be cast out (v28). Jesus however was able to cast out the demon in the mute man, thus proclaiming he is the Messiah (see v 23 – the Son of David is a reference to the Messiah).
The Pharisees, who wanted a reason to reject the Messiahship of Jesus, stated he had cast out the demon by the power of Satan (see also Matthew 9:34). To do this Jesus himself would needed to be demon possessed, or be acting on Satan's behalf. Jesus points out how illogical their claim is – no ruler can be divided against himself and therefore as Satan did not have a divided kingdom it was impossible that Satan was acting against himself. Therefore, they had to accept that Jesus, in the power of the Holy Spirit, had cast out the demon, in the manner the Messiah would, without first seeking the demon's name. The response of the Jewish leaders leads Jesus to proclaim that anyone that speaks against the Holy Spirit will not be forgiven.
Jesus had been baptised by the Holy Spirit (Matthew 3:16) and he did his miracles by the power of the Holy Spirit. Since the Pharisees refuse to believe that Jesus, acting in power of the Holy Spirit, had cast out the demons in the mute man, Jesus judges the Pharisees, indicting them as blasphemers of the Holy Spirit. This was because they proclaimed that Satan rather than the Holy Spirit had removed the demon from the man – making God equal to Satan which is blaspheme. So we see in this example, the Pharisees representing Israel, blaspheme the Holy Spirit and therefore could not be saved. Jesus' judgement is found in verses 30-37. Luke sums up the issue: "But the Pharisees and lawyers rejected the will of God for themselves" (Luke 7:30). Due to this unpardonable sin, that fate of Israel was sealed, and pronounced by Jesus, which was fulfilled in A.D. 70. The outcome makes logical sense: the very person of Christ who had come to save Israel, who was the Messiah, is rejected by Israel by saying he was not the Messiah and was demon possessed (Luke 7:20, John 8:48, Matthew 12:24). It was as if the rescue boat is repulsed by those on board a sinking vessel. The ship sinks and the rescue boat that is rejected cannot save the passengers as it has been sent away. Therefore, having rejected their saviour, that generation of Israel was judged; the temple was destroyed and Jerusalem sacked with the end result being the Jews dispersed over the whole world and the loss of the land until 1948.
Note that the extent of this sin was the whole generation and is unique to the time of Jesus – it cannot be repeated today. Jesus highlights that it was the generation of people at the time Jesus walked this earth that had committed the sin. In response to the scribes and Pharisees demanding a sign that Jesus was the Messiah (when he had just demonstrated by casting out demons in a blind and mute man – a sign recognised in the Jew's own traditions), Jesus responds using the word generation, that is, the collective of Israel, four times (highlighted in the text below): "But He answered and said to them, 'An evil and adulterous generation seeks after a sign, and no sign will be given to it except the sign of the prophet Jonah. For as Jonah was three days and three nights in the belly of the great fish, so will the Son of Man be three days and three nights in the heart of the earth. The men of Nineveh will rise up in the judgment with this generation and condemn it, because they repented at the preaching of Jonah; and indeed a greater than Jonah is here. The queen of the South will rise up in the judgment with this generation and condemn it, for she came from the ends of the earth to hear the wisdom of Solomon; and indeed a greater than Solomon is here. 'When an unclean spirit goes out of a man, he goes through dry places, seeking rest, and finds none. Then he says, 'I will return to my house from which I came.' And when he comes, he finds it empty, swept, and put in order. Then he goes and takes with him seven other spirits more wicked than himself, and they enter and dwell there; and the last state of that man is worse than the first. So shall it also be with this wicked generation.'" Matthew 12:39-45 NKJV.
Rejection of the Holy Spirit and His revelation of truth (John 16:8-11) is blasphemy.
Those that state God is a liar in regard the work of the Lord Jesus Christ, and therefore rejects the gospel, blasphemes the Holy Spirit.
Rejecting the Gospel is blasphemy; in essence it rejects the work of the Holy Spirit.
Blasphemy can only be committed by an unsaved person, one who does not believe in the Lord Jesus Christ.
Jesus proclaims that: ">Therefore I say to you, every sin and blasphemy will be forgiven men, but the blasphemy against the Spirit will not be forgiven men. Anyone who speaks a word against the Son of Man, it will be forgiven him; but whoever speaks against the Holy Spirit, it will not be forgiven him, either in this age or in the age to come." Matthew 12:31-32 NKJV
Blaspheming means to slander God, or put another way, to speak evil of God. Jesus Christ was able to forgive those that blasphemed him – the efficacy of the work he did on the cross is sufficient for every sin including blaspheme. The Apostle Paul notes that he himself was as evil as one could get (he organised the death of Christians), and yet the blood of Christ, shed on the cross was sufficient to save him (1 Timothy 1:15). Furthermore, Jesus Christ desires that all men are saved and come to the knowledge of the truth (1 Timothy 2:4). However, if one attributes to Satan the miracles that Jesus did, that person is wilfully opposing God and thus the Holy Spirit. Such a person cannot be brought to repentance. It is the Holy Spirit that brings a person to God ("No one can come to Me unless the Father who sent Me draws him; and I will raise him up at the last day." John 6:44), and if the Holy Spirit is rejected, that person can never approach God. The rescue boat is sent home, and the passengers will perish.
If you are a believer in Jesus the Son of God (i.e. Yeshua the Messiah) you are saved and cannot therefore commit this sin ("And brought them out, and said, Sirs, what must I do to be saved? And they said, Believe on the Lord Jesus Christ, and thou shalt be saved, and thy house." Acts 16:30-31). The Holy Spirit stands as your guarantee that you are saved (2 Corinthians 1:22, 2 Corinthians 5:5, Ephesians 1:14, Galatians 4:6). Furthermore, there is no need to worry that you may have committed this act – you cannot be unsaved. Those that do reject Christ have no concern about whether they are saved or unsaved, and clearly this is not you, if you are worried about whether you are indeed saved.
If you are not a believer and reject Jesus Christ as the Saviour, you cannot be saved. It is no different from rejecting the rescue boat coming to save a drowning person. In rejecting that fact that Jesus is able to save you, you are essentially crediting the work of God to the devil (or yourself), which is unpardonable: judgement will come. The result of rejecting the Messiah is a conscience that is hardened against the will of God, and no amount of prompting by the Holy Spirit (John 18:7) will lead that person to repent and confess their sin (Luke 14:3&5,15:10, 24:47, Romans 2:4, 2 Corinthians 7:10 etc.). Jesus goes on to claim that the true state of people can be told by the words that come out of their mouths (Matthew 12:33-37) – rejecting Christ shows the true state of the person.