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. According to Black's Law Dictionary, blasphemy is “the written or oral reproach of God, His name, attributes, or religion.” This is similar to slander, which is “malicious oral lies that harm a reputation,” and libel, which is “slander through writing or another type of recording or transmission.” Blasphemy is slander and libel directed at God.
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. Israelites revered the name of God so highly that they refused to pronounce it and removed some of the letters when they wrote it, leaving only the unpronounceable YHWH. 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?" (verse 20). Sennacherib committed blasphemy by assuming Israel’s God was on a par with 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 (37:4, 17). And that’s exactly what God did. Verses 36-37 explain, "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 (verse 38).
Followers of God are also responsible to make sure their behavior doesn't incite others to blaspheme God. In 2 Samuel 12:14, the prophet Nathan explained that David’s adultery with Bathsheba and consequent murder of Uriah gave God's enemies reason to doubt God's holiness. David's sin led others to blaspheme. As a result, God took away the good that came of the sin—a baby boy. The holiness of God was vindicated, and the blasphemy was silenced.
In Romans 2:17-24, Paul scolds those who claim to be saved through the law and yet still sin. Using Nathan’s words to David, Paul tells them "the name of God is blasphemed among the Gentiles because of you" (verse 24). In 1 Timothy 1:20 Paul explains that he had abandoned two Greeks to Satan so they would “be taught not to blaspheme.”
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, contumelious 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).
The fact is, every time we do or say something that gives others a false representation of the glory, holiness, authority, and character of God, we commit blasphemy. Every time we misrepresent our position as children of God, we are damaging His reputation. Fortunately, Jesus forgives even the sin of blasphemy. Peter attacked Jesus' purpose (Matthew 16:22), Paul tried to make others blaspheme (Acts 26:9-18), and Jesus' own brothers thought He was insane (Mark 3:21). All repented, and all were forgiven.