What does it mean to blaspheme? What is blasphemy?

from the old testament

• Blasphemy is derisive language or any insult against God, His character, or His system of proper worship. It is addressed in the Old Testament through the third commandment, which forbids taking the Lord’s name in vain (Exodus 20:7), in the pre-Mosaic law forbidding the murder of those made in God’s image (Genesis 9:6), and in the explicit command not to blaspheme God’s name (Leviticus 24:16).
• Blasphemy was punishable by death under the Old Testament law (Leviticus 24:16)
• David considered it a grave offence to disrespect God by harming His anointed ruler (1 Samuel 24:6), even if that anointed one was trying to kill him.
• Blasphemy involves showing utter scorn and disrespect for God (2 Samuel 12:14).

from the new testament

• The New Testament shows that blasphemy is any disrespect or slander against God, His character, or His work, including Jesus (Mark 3:21) and the Holy Spirit (Matthew 12:31–32; Hebrews 6:4–6).
• The Pharisees accused Jesus of blasphemy when He claimed to be God (Matthew 26:65). It would have been blasphemy if Jesus was not actually God.
• Ananias and Sapphira committed a type of blasphemy when they discounted God's omniscience and holiness and pretended to donate more to the church than they actually had (Acts 5:1–10).
• Paul once persecuted Christians, denying Jesus, which would be considered blasphemy (Acts 26:9–18).
• Even calling Jesus “out of His mind,” as His family did, can be considered blasphemy (Mark 3:21).
• The ultimate blasphemy is rejecting the conviction of the Holy Spirit, hardening the heart, and refusing Christ. It is the complete rejection of Christ, telling the Holy Spirit—who is convicting the soul of sin—that He is wrong, that Jesus is not God or their Savior, even though they have become convinced of the truth (Hebrews 6:4–6). This is the only unpardonable sin and leads to spiritual death (John 3:16–18, 36; Romans 3:23). A true Christian cannot blaspheme God in this way, showing that those who do reject God or those who walk away from the faith were never saved to begin with.

implications for today

"Disrespectful speech or representation of God" is a broad definition of what it means to blaspheme, and it's very easy to fall into. God is Creator of the universe, absolute Sovereign, and our fallen minds don't have the capacity to fully comprehend Him—how can we accurately represent Him? Is inaccuracy tantamount to disrespect? This very concern is what drove the Israelite scholars to condense God's name to the unpronounceable "YHWH." While a true Christian can never actually blaspheme God by rejecting Him, we can fail to honor His name, misrepresent His character, or speak carelessly about His work. Therefore, we should take seriously the call to reflect God’s holiness in our words and actions, speaking truthfully about Him and reflecting Him to others with reverence. This is only possible as we abide in Him and allow the Holy Spirit to grow and transform us into Christlikeness, convicting us when we do not live according to His ways or misrepresent Him. May we wholeheartedly seek to glorify God instead of inadvertently disrespecting Him.