The Greek noun blasphēmia means blasphemy, slander, or defamation — the act of speaking injuriously about someone or something, especially about God or sacred things. In the New Testament it can refer both to blasphemy against God and to slander against persons. The word combines blapto (to harm) and phēmi (to speak) — harmful speech.
Jesus declared that blasphemy against the Son of Man can be forgiven, but blasphemy against the Holy Spirit cannot (Matthew 12:31–32). This "unforgivable sin" has perplexed Christians throughout history. The context reveals its nature: the Pharisees, confronted with undeniable evidence of the Spirit's work in Jesus, deliberately attributed it to Satan — a willful, hardened rejection of the Holy Spirit's testimony. This is not a momentary doubt but a settled, persistent stance of calling the Spirit of God evil. The blasphemy against the Spirit is the final refusal to receive the only One who can lead a person to repentance.