Speech that is abusive to humans or derogatory to God. Blasphemy against humans occurs when people speak words harmful to one another (Matt. 15.19; Col. 3.8; NRSV: “slander”). Blasphemy also occurs when a person speaks against God in a way that fails to recognize the sacredness and honor of God's person and name. According to Leviticus 24.10–16, it was punishable by death.
The gospel writers also describe Jesus' opponents as blasphemous when they mocked him (Mark 15.29; Luke 22.65). Similarly, 1 Timothy says that Paul blasphemed Jesus when he persecuted the church (1.13), and that those who deserted the gospel were also guilty of blasphemy (1.20).
In a passage that has elicited much debate, the synoptic Gospels (Mark 3.28–29 par.) speak of blasphemy against the (Holy) Spirit as a sin that cannot be forgiven. The context indicates that this sin is not committed unintentionally by Jesus' followers, but is ascribed to the adversaries of Jesus, who had attributed his success to an evil spirit.
Paul L. Bremer