It’s not ‘sin’ against the Holy Spirit which is unforgivable – it is ‘blasphemy’ and ‘speaking against’ which are specified.
Matthew 12:22 Then was brought unto him one possessed with a devil, blind, and dumb: and he healed him, insomuch that the blind and dumb both spake and saw. 23 And all the people were amazed, and said, Is not this the son of David? 24 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 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.
If we believe the doctrine of the Trinity, there can be no satisfactory answer to the question. Christ’s statement - that blasphemy and speaking against himself (the second person of the Trinity) is forgivable (in this world and the next) whereas blasphemy and speaking against the Holy Spirit (the third person of the Trinity) is unforgivable - destroys the teaching of co-equality, and with it, the false doctrine.
If we believe the truth – that the Holy Spirit is not a person but rather the power of God – the answer is fairly straightforward.
That the Holy Spirit is the power of God is clearly testified in Luke 1:35
Luke 1:35 And the angel answered and said unto her, The Holy Spirit shall come upon thee, and the power of the Highest shall overshadow thee: therefore also that holy thing which shall be born of thee shall be called the Son of God.
The Holy Spirit = the power of the Highest
There is no power in the whole of creation other than that of God. Everything was created and is sustained by his Spirit. God alone has the power to work miracles. Miracles are worked by his Holy Spirit.
Jesus acknowledged that God did the works that he performed.
Matthew 12:28 But if I cast out devils by the Spirit of God, then the kingdom of God is come unto you.
Luke 11:20 But if I with the finger of God cast out devils, no doubt the kingdom of God is come upon you.
John 14:10... the Father that dwelleth in me, he doeth the works.
Peter confirms that God did the works
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 miracles wonders and signs were done by the power of God – the Holy Spirit.
The miracles were God’s testimony to the truth spoken by his son.
Blasphemy against the Holy Spirit is clearly a sin against God.
What makes this particular sin unforgivable is that miracles which God alone can do are ascribed to the power of a fictitious entity – Beelzebub – so the scribes and Pharisees are guilty of both contradicting the active testimony of God and gross idolatry.
This is the reason why blasphemy against undisputable miracles worked by the power of God – the Holy Spirit – is unforgivable; both in this present world and in God’s kingdom when Christ returns.
The perpetrators have no excuse; because there is no other power that can work miracles. Beelzebub, Satan, and demons have no literal existence; and consequently have no power to work miracles.
I hope you have found this helpful.