According to the Bible, what is blasphemy?
Blasphemy is a term that generally denotes abusive or defamatory language concerning God. However, the Hebrew and Greek terms thus translated are used also for such language with respect to man.
In translating these terms, our English versions generally reserve “blaspheme” and “blasphemy” for instances where God’s name or character is defamed (see Leviticus 24:11 and James 2:7). The defamation may be direct (Acts 6:11) or implied—as in attributing evidence of divine power to the devil (Matthew 12:31). It also may be in claiming divinity or divine prerogatives (Matthew 9:1–6; 26:65; John 10:33, 36).
Moreover, blasphemy is more directly assuming the authority of God in the form of forgiving sins (Mark 2:7), persecuting others in the name of God (1 Timothy 1:12–13), or claiming to be, and/or usurping the divine offices of God (John 10:33).
Then there is at least one instance where those who claimed to be followers of God, but in reality were of the church of Satan, committed blasphemy (Revelation 2:9). This would identify professed Christians who act like the devil as blasphemers, too.