Blasphemy is showing disrespect for God. Under the Law of Moses, anyone who blasphemed was to be executed by stoning (Leviticus 24:16).
“Blasphemy against the Holy Spirit” is only mentioned in the gospels:
Matthew 12:31-32. “Therefore I tell you, every sin and blasphemy will be forgiven people, but the blasphemy against the Spirit will not be forgiven. 32 And whoever speaks a word against the Son of Man will be forgiven, but whoever speaks against the Holy Spirit will not be forgiven, either in this age or in the age to come.”
Mark 3:28-30. “Truly, I say to you, all sins will be forgiven the children of man, and whatever blasphemies they utter, 29 but whoever blasphemes against the Holy Spirit never has forgiveness, but is guilty of an eternal sin”– 30 for they were saying, “He has an unclean spirit.”
Luke 12:10. “And everyone who speaks a word against the Son of Man will be forgiven, but the one who blasphemes against the Holy Spirit will not be forgiven.”
Here, “blasphemy against the Holy Spirit” appears to mean consciously and deliberately showing contempt for the power of God.
In Matthew 12 and Mark 3, the scribes and Pharisees had seen amazing evidence that Jesus was performing miracles by the power of the Holy Spirit, but they claimed instead that the miracles were done by the power of “Beelzebul, the prince of demons” (Matthew 12:24). The blasphemy was to ascribe the power of God’s Holy Spirit to a pagan deity.
This blasphemy against the Holy Spirit seems to have been much more considered and deliberate than ordinary blasphemy. The Pharisees knew that what they said was not true, yet said this to try to discredit Jesus for political reasons. Hence, it was a very deliberate and conscious strategy, not something done in the heat of the moment, or in ignorance.