Blasphemy is irreverent speech or action regarding something held to be sacred. What is considered blasphemous and how (or if) it should be punished varies widely with different religious traditions.
What Constitutes blasphemy?
Depending on the particular denomination, most religions do not consider phrases such as "for God's sake" or "God damn" blasphemous. Some only consider them blasphemous if they are not meant literally; i.e. "God damn" just as an expression and not God actually damning something. Others see this as taking God's name in vain, and thus blasphemy.
Islam typically does not consider such expressions to be blasphemous unless they refer to something other than the Islamic god as being divine, i.e. "Jesus Christ!" However, speaking negatively of God or prophets is considered blasphemous.
Blasphemy in the Old Testament
According to Leviticus 24:16, blasphemers "shall surely be put to death."
Blasphemy in the New Testament
Luke 12:10 describes blasphemy as unforgivable. However, interpretations of this passage vary widely as to what constitutes blasphemy against the Holy Spirit and whether it can be forgiven. The Catholic Church, for example, has prayers specifically for forgiveness of blasphemy.
Blasphemy in Islam
Islamic tradition holds that blasphemers will not enter heaven unless they repent before they die.
In the United States, blasphemy is protected speech under the First Amendment. However, prior to the Fourteenth Amendment, the Bill of Rights was not considered to apply to state governments, and several states had laws against blasphemy on their books. These laws have been ruled unconstitutional.
The United Kingdom had blasphemy laws on the books until 2008. They specifically dealt with blasphemy against Christianity.
In Pakistan, blasphemy is punishable by life imprisonment.