The Greek verb blasphemeō (βλασφημέω) means to speak irreverently or sacrilegiously against God, to slander or defame. The related noun blasphemia is transliterated directly into English as 'blasphemy.' In classical Greek it meant to speak ill of anyone, but in the New Testament it primarily refers to dishonoring the name and nature of God.
Blasphemy is treated with extreme gravity in Scripture. Jesus declared that blasphemy against the Holy Spirit — attributing the works of the Spirit to Satan — is the unforgivable sin (Matthew 12:31–32). The religious leaders accused Jesus of blasphemy for claiming divine authority (Mark 14:64), ironically committing the very sin they charged Him with. Paul acknowledged having previously blasphemed Christ (1 Timothy 1:13), yet received mercy through ignorance. Believers are warned not to give cause for God's name to be blasphemed among unbelievers (Romans 2:24), making godly living a matter of God's honor.