It is
because they said he has an unclean spirit that he said blasphemy against the Holy Spirit is an eternal sin:
28“Truly I say to you, all sins shall be forgiven the sons of men, and whatever blasphemies they utter; 29but whoever blasphemes against the Holy Spirit never has forgiveness, but is guilty of an eternal sin”— 30because they were saying, “He has an unclean spirit.”
That means they committed that sin. And that it's a sin that can not be overlooked in any way shape or form. You can be guilty of some sins and it not matter to your eternal salvation,
but this is not one of them. Die in that state and you're toast.
