As they went on their rampage, the men who killed 12 people in Paris last week yelled that they had "avenged the Prophet". They follow in the path of other terrorists who have bombed newspaper offices, stabbed a film-maker and killed writers and translators, all to mete out what they believe is the proper Quranic punishment for blasphemy.
But, in fact, the Quran prescribes no punishment for blasphemy. Like so many of the most fanatical and violent aspects of Islamic terrorism today, the idea that Islam requires that insults against the Prophet Muhammad be met with violence is a creation of politicians and clerics to serve a political agenda.
One holy book is deeply concerned with blasphemy: The Bible. In the Old Testament, blasphemy and blasphemers are condemned and prescribed harsh punishment. The best-known passage on this is Leviticus 24:16: "Anyone who blasphemes the name of the Lord is to be put to death. The entire assembly must stone them. Whether foreigner or native-born, when they blaspheme the Name they are to be put to death."
In contrast, the word blasphemy appears nowhere in the Quran. (Nor, incidentally, does the Quran anywhere forbid creating images of Muhammad, though there are commentaries and traditions - "hadith" - that do, to guard against idol worship.) Islamic scholar Maulana Wahiduddin Khan has pointed out that "there are more than 200 verses in the Quran which reveal that the contemporaries of the prophets repeatedly perpetrated the same act which is now called 'blasphemy or abuse of the Prophet'... but nowhere does the Quran prescribe the punishment of lashes, or death, or any other physical punishment".
On several occasions, Muhammad treated people who ridiculed him and his teachings with understanding and kindness. "In Islam," scholar Khan says, "blasphemy is a subject of intellectual discussion rather than a subject of physical punishment".
Somebody forgot to tell the terrorists. But the gruesome and bloody belief they have adopted is all too common in the Muslim world, even among so-called moderate Muslims - that blasphemy and apostasy are grievous crimes against Islam and should be punished fiercely. Many Muslim-majority countries have laws against blasphemy and apostasy - and in some places, they are enforced.
Pakistan is now the poster child for the anti-blasphemy campaign gone wild. In March, at least 14 people were on death row in that country, and 19 were serving life sentences, according to the US Commission on International Religious Freedom.
The owner of the country's largest media group has been sentenced to 26 years in prison because one of his channels broadcast a devotional song about Muhammad's daughter while re-enacting a wedding. (Really.)
And Pakistan is not alone. Bangladesh, Malaysia, Egypt, Turkey and Sudan have all used blasphemy laws to jail and harass people. In moderate Indonesia, 120 people have been detained for this reason since 2003. Saudi Arabia forbids the practice of any religion other than its own Wahhabi version of Islam.
The Pakistani case is instructive because its extreme version of anti-blasphemy law is relatively recent and a product of politics. Muhammad Zia ul-Haq, Pakistan's president during the late 1970s and 1980s, wanted to marginalise the democratic and liberal opposition, and he embraced Islamic fundamentalists, no matter how extreme. He passed a series of laws Islamising Pakistan, including a law that recommended the death penalty or life imprisonment for insulting Muhammad in any way.
When governments try to curry favour with fanatics, eventually the fanatics take the law into their own hands. In Pakistan, they have killed dozens of people whom they accuse of blasphemy, including a brave politician, Salmaan Taseer, who dared to call the blasphemy law a "black law".
We should fight the Paris terrorists. But we should also fight the source of the problem. It's not enough for Muslim leaders to condemn people who kill those they consider as blasphemers if their own governments endorse the idea of punishing blasphemy at the very same time. The US religious freedom commission and the United Nations Human Rights Committee have both declared that blasphemy laws violate universal human rights because they violate freedom of speech and expression. They are correct.
In Muslim-majority countries, no one dares to dial back these laws. In Western countries, no one confronts allies on these issues. But blasphemy is not a purely domestic matter, of concern only to those who worry about countries' internal affairs. It now sits on the bloody crossroad between radical Islamists and Western societies. It cannot be avoided any more.
Western politicians, Muslim leaders and intellectuals everywhere should point out that blasphemy is something that does not exist in the Quran and should not exist in the modern world.