Terrorists slaughtered a roomful of French cartoonists in Paris yesterday.

That sentence is so absurd and appalling it is difficult to accept as a statement of fact. The murderers of the staff of Charlie Hebdo, a satirical magazine that had frequently lampooned the Prophet Muhammad and Islam, reportedly shouted, "Allah is revenged." So as France mourns its dead, Western Europe is once again forced to consider how to preserve the norms of a secular society in which religion can be mocked. This is all the more difficult when the most vital religious force in that society is a poor, devout, and mostly immigrant minority.

We often talk about secularism as if it is an obvious principle that can be easily applied to all nations, governments, and religions. It is anything but. Attempts by Westerners to impose the ideals of secularism on Muslim immigrants have only exposed the fact that restrictions on religion cannot be applied with a broad brush.

France, in particular, has bent over backward to prove that its version of secularism is blind. The country banned the wearing of "large crosses" in schools and public institutions to give a veneer of impartiality to its real goal of banning Islamic veils in the same places. Confronted with an Islamic tradition that gave offense, France invented a Christian analogue to ban along with it, a phony gesture at neutrality that has been imitated elsewhere.

But secularism is a political, legal, and cultural project that goes back centuries, with roots in the "two swords" doctrine of medieval Christianity. The target of modern secularism was (and still is, really) the Christian Church, which it sees as the instigator and vehicle of majoritarian prejudice. Secularism aims to prevent Europe's wars of religion from ever happening again, and to contain the power of Europe's churches when it comes to politics and culture.

It encourages a special disgust with religious violence in history. Ditto religious motivations in democratic politics.

Modern secularism creates a taboo against distinguishing between religions. To judge one in any way superior to another is a step away from enlightenment and civilization, and a step toward the Thirty Years War. You are allowed to mock and hate Islam, but must make a show of doing it "equally" to other religions. You are also allowed to respect religion, but the same principle applies. This brigade of pieties exists to prevent acts of hatred and to stifle prejudice, but it inadvertently guards against any intelligent conversation about religion.

After yesterday's attack, many liberals rushed to affirm the right to offend and to blaspheme. They allied themselves with the legacy of Christopher Hitchens, who could write acidly about Islam but also played with the rather illiberal idea of categorizing religious education as child abuse. These voices are leaning hard on the secularist idea that religious people cannot be allowed a veto on free speech, even in an age in which we discuss offering trigger warnings to those far more privileged than the men in French banlieues.

Other progressives feel that mocking Muslims is a form of racism. Or, more circumspectly, that the cartoonists of Charlie Hedbo were somehow unsporting. This says a lot about secularism: The point is to make sure France isn't ruled by contraception-deploring Ursuline nuns, not to bring a minority to heel. Secularism is not about religion per se, but a tool for rearranging the distribution of power.

The taboos of secularism interlock in other odd ways. Modern Western secularists feel no anxiety whatsoever when they encounter harsh criticism and satire of Christianity. But if you offer a particularly barbed remark about Islam among the enlightened, someone will ask you to politely agree that Christianity is just as bad. And ironically, this instinct to protect the powerless is a leftover instinct of Christian civilization, which put sayings like "the last shall be first, and the first shall be last" at the heart of its worship and moral imagination.

We used to say of comedians, "He can make that joke because he's Jewish." In this respect, the Western world's comfort with attacking Christianity is an inadvertent admission that Christianity is "our" religion. And so it elicits from us none of the respect, deference, or fear we give to strangers. Viewed this way, secularism looks less like universal principle than a moral and theological critique derived from Christian sources and pitched back at Christian authorities.

The great irony of Islam's continued clashes with the Western way of life — whether its widespread riots over a YouTube video or the murderous actions of a crazed minority— is that it has revealed, to the surprise of everyone but Pope Emeritus Benedict, that modern secularism is a kind of epiphenomenon of Christendom.

To borrow from G.K. Chesterton, secularism is the second fermentation, where the wine of Christianity becomes the vinegar of laïcité. Force either of them into the mouth of a Muslim guest, and he will spit it out.