Question: "What is blasphemy? What does it mean to blaspheme?"
Answer: To blaspheme is to speak with contempt about God or to be defiantly irreverent. Blasphemy is verbal or written reproach of God's name, character, work, or attributes.
Blasphemy was a serious crime in the law God gave to Moses. The Israelites were to worship and obey God. In Leviticus 24:10–16, a man blasphemed the name of God. To the Hebrews, a name wasn’t just a convenient label. It was a symbolic representation of a person’s character. The man in Leviticus who blasphemed God’s name was stoned to death.
Isaiah 36 tells the story of Sennacherib, king of Assyria, and his attempt to demoralize Jerusalem before he attacked. After pointing out Assyria's many victories, he says, "Who of all the gods of these countries have been able to save their lands from me? How then can the LORD deliver Jerusalem from my hand?" (Isaiah 36:20). Sennacherib committed blasphemy by assuming Israel’s God was equal to the false gods of the surrounding nations. The king of Judah, Hezekiah, points out this blasphemy in his prayer to God, in which he asks that God deliver them for the purpose of defending His own honor (Isaiah 37:4, 17). And that’s exactly what God did. Isaiah 37:36-37 explains, "Then the angel of the LORD went out and put to death a hundred and eighty-five thousand in the Assyrian camp. When the people got up the next morning—there were all the dead bodies! So Sennacherib king of Assyria broke camp and withdrew. He returned to Nineveh and stayed there." Later, Sennacherib was murdered in the temple of his god Nisroch (Isaiah 37:38).
Followers of God are also responsible to make sure their behavior doesn't incite others to blaspheme God. In Romans 2:17-24, Paul scolds those who claim to be saved through the law and yet still live in sin. Using Isaiah 52:5, Paul tells them, “God’s name is blasphemed among the Gentiles because of you” (verse 24). In 1 Timothy 1:20 Paul explains that he had abandoned two false teachers to Satan so they would “be taught not to blaspheme”; thus, promulgating false doctrine and leading God’s people astray is also a form of blasphemy.
Jesus spoke of a special type of blasphemy—blasphemy against the Holy Spirit—committed by the religious leaders of His day. The situation was that the Pharisees were eyewitnesses to Jesus’ miracles, but they attributed the work of the Holy Spirit to the presence of a demon (Mark 3:22-30). Their portrayal of the holy as demonic was a deliberate, insulting rejection of God and was unforgiveable.
The most significant accusation of blasphemy was one that happened to be completely false. It was for the crime of blasphemy that the priests and Pharisees condemned Jesus (Matthew 26:65). They understood that Jesus was claiming to be God. That would, indeed, be a reproach on God's character—if it wasn't true. If Jesus were just a man claiming to be God, He would have been a blasphemer. However, as the Second Person of the Trinity, Jesus could truthfully claim deity (Philippians 2:6).
Fortunately, Jesus forgives even the sin of blasphemy. Paul was a blasphemer (1 Timothy 1:13) and tried to make others blaspheme (Acts 26:11). Jesus’ own brothers thought He was insane (Mark 3:21). All repented, and all were forgiven.
Blasphemy, by definition, is both deliberate and direct. That being the case, a believer in Jesus Christ will not/cannot commit blasphemy. Even so, we should be careful to reflect God's holiness and never misrepresent the glory, authority, and character of God.