What was the blasphemy against the Holy Spirit
There are many who don't understand what Jesus meant when he warned that blasphemy against the Holy Ghost was unforgivable. To understand this statement it is necessary for us to place ourselves in the same circumstances in which such statement was uttered, and to consider God's law against blasphemy. In Lev 24: 16 God orders that anyone who blasphemes be executed. It was to say that the blasphemy against the Holy Spirit was not the only unfogivable one, because the blaspheme against God the Father was not fogiven either.
"And he that blasphemeth the name of the LORD, he shall surely be put to death, and all the congregation shall certainly stone him; as well the stranger, as he that is born in the land, when he blasphemeth the name of the LORD, shall be put to death." ( Lev 24: 16 )
No Hebrew in all Israel, no matter how renegade he was, would dare to blaspheme God. Not even those who didn't believe in God would dare to blaspheme Him, because they knew that they would immediately have to pay the blasphemy with their life. Everyone who would hear them would stone them. Therefore, when Our Lord Jesus Christ said that all manner of blasphemy was going to be forgiven, he was not referring to those who would blaspheme God the Father, Jehovah, the God of the Hebrews, because such a blaspheme was uttered by none, it was something unthinkable. Nobody would dare to blaspheme Jehovah God, much less publicly, as these Pharisees blasphemed the Holy Ghost. Jesus was referring to blasphemies against himself.
"31 Wherefore I say unto you: All manner of sin and blasphemy shall be forgiven unto men, but the blasphemy against the Holy Ghost shall not be forgiven unto men. 32 And whosoever speaketh a word against the Son of man, it shall be forgiven him; but whosoever speaketh against the Holy Ghost, it shall not be forgiven him, neither in this world, neither in the world to come." ( Mt 12: 31-32 )
Therefore, disregarded the possibility that those Pharisees were blaspheming against God the Father, let's exam the possibility that the blaspheme was against the Divine Son of God. In Mtt 12: 32; Mk 3: 29 and Lk 12: 10, Jesus says that any blaspheme that anyone utters against him would be forgiven. Since those people saw Jesus as just a man, like themselves, the fact that they would blaspheme him didn't have the severity of blaspheming God the Father. From their point of view Jesus was just a human being, that is why insults and blasphemes against him could be forgiven. This was product of their ignorance; they didn't believe that he was a Divine Being. It is more evident in Mt 12: 32 and Lk 12: 10, where Jesus talks about himself as "the Son of Man", consequently, those who saw him as just a man, didn't commit the unforgivable sin, when they blasphemed Jesus.
"28 Verily I say unto you: All sins shall be forgiven unto the sons of men, and blasphemies wherewith soever they shall blaspheme, 29 but he that shall blaspheme against the Holy Ghost hath never forgiveness, but is in danger of eternal damnation. 30 Because they said: He hath an unclean spirit." ( Mk 3: 28-30 )
"And whosoever shall speak a word against the Son of man, it shall be forgiven him: but unto him that blasphemeth against the Holy Ghost it shall not be forgiven." ( Lk 12: 10 )
Let's now look at all these passages, in order to figure out who the Pharisees were blaspheming against. According to Mk 3: 30, what Jesus considered a blasphemy was the fact that they said the Spirit that Jesus had, was an unclean spirit. Those people were talking of the Divine Being whose anointment allowed Jesus to make miracles. Their case was different from the previous one, because they had to be fully aware that the Spirit that allowed such miracles to be performed, had to be a Divine Being. Despite this evident conclusion they chose to blaspheme this Divine Being, calling him Beelzebub, as we see in Mt 12: 24; Mk 3: 22 and Lk 11: 15. They knew that no demon could cast out another demon. They knew that Satan could never open the eyes of a blind, resurrect a dead person, nor make whole those sick with palsy. That's why they couldn't be forgiven, because they knew to whom they were blaspheming, and they persisted in doing so.
This was not the case of an ignorant or an atheist, who did not know the meaning of what they were uttering, or didn't know or didn't believe that such a divine being existed. This was the case of people who knew perfectly well whom they were blaspheming against, and even so, they wished to blaspheme him. They did so in order to get the "benefit" of casting doubts over the miracles performed by Jesus Christ, because they perceived him as their competitor in religion.
"But when the Pharisees heard it, they said: This fellow doth not cast out devils, but by Beelzebub the prince of the devils." ( Mtt 12: 24 )
"And the scribes which came down from Jerusalem said: He hath Beelzebub, and by the prince of the devils casteth he out devils." ( Mk 3: 22 )
"But some of them said: He casteth out devils through Beelzebub the chief of the devils." ( Lk 11: 15 )
That is why the blasphemy against the Holy Ghost shall not be forgiven unto men.