Aren't all sins forgiveable?
Matthew 12:24-32 But when the Pharisees heard it, they said, This fellow doth not cast out devils, but by Beelzebub the prince of the devils... 31 Wherefore I say unto you, All manner of sin and blasphemy shall be forgiven unto men: but the blasphemy against the Holy Spirit 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 Spirit, it shall not be forgiven him, neither in this world, neither in the world to come.
Mark 3:22-30 And the scribes which came down from Jerusalem said, He hath Beelzebub, and by the prince of the devils casteth he out devils… 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 Spirit hath never forgiveness, but is in danger of eternal damnation: 30 Because they said, He hath an unclean spirit.
Luke 12:10 And whosoever shall speak a word against the Son of man, it shall be forgiven him: but unto him that blasphemeth against the Holy Spirit it shall not be forgiven.
The casting out of demons was done by the Holy Spirit (the power of God). The Pharisees told the people that Jesus was doing it by the power of the fictitious Beelzebub. They were therefore blaspheming the Holy Spirit.
Blasphemy is a specific kind of sin. God used his power (the Holy Spirit) to show that what Jesus was teaching was from God.
Acts 2:22 Ye men of Israel, hear these words; Jesus of Nazareth, a man approved of God among you by miracles and wonders and signs, which God did by him in the midst of you, as ye yourselves also know:
The Pharisees were saying that the miracles were done not by God, but by Beelzebub. They were blaspheming the power of God.
What Jesus says is that if they blaspheme him it is forgivable, but if they blaspheme the power of God, their blasphemy is against God himself and will not be forgiven. It is not sin in general but the specific sin of blasphemy.
It does show that God is not a trinity where the Father and the Son and the Holy Spirit are co-equal persons. Blasphemy against the Son is forgivable but ascribing the demonstration of the power of God to a fictitious entity called Beelzebub will not be forgiven. This because God is a jealous God, whose glory is not to be ascribed to another.
Exodus 34:14 For thou shalt worship no other god: for the LORD, whose name is Jealous, is a jealous God:
Isaiah 42:8 I am the LORD: that is my name: and my glory will I not give to another, neither my praise to graven images.
Beelzebub began as a god of Ekron.
2 Kings 1:16 And he said unto him (Ahaziah), Thus saith the LORD, Forasmuch as thou hast sent messengers to enquire of Baalzebub the god of Ekron, is it not because there is no God in Israel to enquire of his word? therefore thou shalt not come down off that bed on which thou art gone up, but shalt surely die.
Ahaziah died because he thought Baalzebub was real and had power to foretell the future. The Pharisees should have known better.
The direct manifestation of the power of God through individuals is not active today, so we cannot blaspheme the Holy Spirit by ascribing the manifestation of God’s power to a false God.
May I suggest that you read: The Holy Spirit (please click on the title)
I hope you find this helpful.