Gilgamesh:
You ask how Christians define "blasphemy against the Holy Spirit" and cite Matthew 12:31.
Preachers are fond of quoting Matthew 12 when citing this story because then the reason for eternal damnation is left open to the preacher’s interpretation of blaspheming the Holy Spirit. Then they can rant and rave regarding all the ways you can be condemned to hellfire and damnation.
Only in Mark 3 is the story amplified to explained why the blasphemer is guilty of eternal sin.
(First Mark reports) 22: And the teachers of the law who came down from Jerusalem said, He is possessed by Beelzebub! By the prince of demons he is driving out demons.
(Then Jesus is quoted as saying) 28: I tell you the truth, all the sins and blasphemies of men will be forgiven them. 29: But whoever blasphemes against the Holy Spirit will never be forgiven; he is guilty of an eternal sin.
(Then Mark writes regarding the why) 30: He said this because they were saying, He has an evil spirit.
So, it seems that anyone who makes false claims regarding Jesus can be forgiven because they may have mistaken Him for a man, but those who claim that the healing work done through Jesus by the Holy Spirit was done by Satan are guilty of eternal sin.
Now this taken together with I came to fulfill the Law may indicate that somewhere in the Law is an admonition to recognize and regard the work of the Holy Spirit and not make false claims regarding it or attribute it to the evil spirits. Maybe it has to do with "consorting with familiar spirits" or some such thing as that. I would look in Leviticus or one of the other Mosaic books to find a connection between the levity of denying God, consorting with spirits, and bearing false witness and such.
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