Blasphemy is any word or act that insults the honor and dignity of another. The Bible sometimes uses the term about a person who reviles another (I Kings 21.10). More often, however, the term is used to describe reviling or speaking disrespectfully of God. The third commandment prohibits blasphemy: "Thou shalt not take the name of the LORD thy God in vain" (Exod. 20.7). The Law of Moses prescribed death by stoning for blasphemy (Lev. 24.10-16). After a person was killed by stoning for blasphemy or idolatry, his body was often hanged from a stake or a tree until sundown as a reminder to the people of the heinousness of his offense. Idolatry, often equated with blasphemy, implied contempt for the Lord should someone worship other gods.

Jews considered it blasphemous to pronounce the sacred name of God, YHWH, the name that God had revealed to Moses at the burning bush on Mount Sinai. Later this name, transliterated Yahweh and that means "I am who I am," was held in such reverence that it was always pronounced "Adonai," another name for God, whenever it was encountered in reading the Scriptures. For Jesus to say to the religious leaders, "Before Abraham was I am," was blasphemous in their eyes, for he used God's sacred, unpronounceable name to refer to himself.

Blasphemy "against the Holy Spirit" (Mark 3.22-30) is crediting the devil with the power Jesus had to cast out demons.