Blasphemy against the spirit boils down to having a level of spiritual understanding in order to even commit it to begin with. Requirements:
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You have to be or have been a believer in God.
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You have to witness acts of the spirit happening.
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You have to attribute those acts to Satan.
It's a tricky thing that requires a level of spiritual understanding that, when committed, it meant to rebuke all that is God. In other words, when Jesus usurped the spiritual leadership of the Pharisees, he was giving the people a way to have a direct relationship with God. When he performed miracles to prove himself, the Pharisees came along and accused him of commanding demons by being one himself.
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.
The Pharisees were jealous because they could not perform the miracles Jesus did. So in their jealousy and hatred for his works, they sinned against the Holy Spirit by declaring Jesus was a prince of Hell, using demonic authority to cast out demons.
That's the full context of the "unforgivable sin". This is a sin that most today wouldn't be capable of because it would require a much higher level of spiritual understanding; something that's really lacking these days among the lost.