Blasphemy (from the Greek blaptein, meaning “to injure,” and pheme, meaning “reputation”) is the sin in which one thinks, speaks, or writes “against God, inwardly or outwardly, words of hatred, reproach, or defiance; in speaking ill of God; in failing in respect toward Him in one’s speech; in misusing God’s name” (Catechism #2148). It is a violation of the Commandment “Thou shalt not take the name of the Lord thy God in vain.”
Using God’s name to curse or swear is the most common form of blasphemy. This includes using the word “God” and combining it with a curse phrase such as “damn it” or “damn you.” Saying the name of Jesus Christ while in anger is blasphemous, as are depictions in so-called art where God or the saints are ridiculed, mocked, or shown great disrespect and dishonor.
Blasphemy also extends to the vandalism or desecration of churches and of holy items, like relics and sacramentals.