The murder last week of the Catholic priest Jacques Hamel as he celebrated morning Mass in a church in the suburb of the French city of Rouen showed that Islamic State is the most aggressively innovative of the serious challenges that democratic societies confront today.

The violent act broke two powerful taboos. The first was that a priest was killed while kneeling at the altar in a chapel. Church sanctuary, recognized as a refuge of safety from the Middle Ages through the 17th century, no longer has any legal force. Yet, its breach still shocks.

Catholic priests have been murdered in large numbers through the ages — at the hands of Protestants (it was mutual), Muslims, communists and Nazis. But the isolated fact of a savage murder on a suburban summer morning stuns by its immediacy, and by being bathed in artificial light by the news media.

The second taboo is age: The priest was 85. He had won permission to continue his duties after the normal retirement age of 75.

Age is no protection against murder by random shooting or by a truck weaving to kill as many people as possible, as in Nice, France, recently. But the nature of these methods of mass murder obscures the individual horror. In Father Hamel’s case, it was vivid — and the method of killing, his throat was cut, a signature of the assassin (for which a synonym is “cutthroat”).

The two young men who carried out the killing filmed it, a bloody selfie for later viewing. Though not by them, because both were subsequently shot dead by police.

In all these ways, IS is brutally post-modernist. This kind of post-modernism, however, should never be compared or confused with the kind of aesthetic post-modernism used to describe the work of, say, the Young British Artists of the 1990s. Among these artists were Damien Hirst with his shark in formaldehyde, titled “The Physical Impossibility of Death in the Mind of Someone Living,” for example, or Tracey Emin with “My Bed.” They took objects, framed them and assumed the media would also frame them — if often to condemn their works as rubbish.

But they were artists-cum-entrepreneurs, who now live peacefully as multimillionaires. The shark in formaldehyde, for example, sold for between $8 million and $12 million.

By contrast, IS forces its murderous art upon the West, and also uses the media. It tells increasingly unsettled Western states that their taboos, conventions, courtesies and, above all, liberal institutions and mechanisms are under attack.

The political scientist Francis Fukuyama labeled the victory of liberal capitalist democracy over Soviet communism as “the end of history.” He didn’t say that nothing else important would happen. He did say, though, that all societies would “end their ideological pretensions of representing different and higher forms of human society.”

Militant Islam, especially the form that IS has decreed it should take, proves this is wrong. It bends all its strength to drag Western culture down to what it views as a “higher form” of human society.

IS’ more monstrous innovation is that its message — “to target Crusader coalition states” in any way possible, with whatever comes to hand — is capable of rapid absorption.

The Nice truck-killer, Mohammed Lahouaiej Bouhlel; Omar Mateen, the Afghan-American who killed 49 at a gay nightclub in Orlando, Florida, in June, and Ali David Sonboly, the German-Iranian who killed nine in Munich last week, all had, at best, tenuous connections to IS. Each probably radicalized himself, over a short period of time, inspired by IS internet propaganda.

IS is fast losing ground and fighters in Iraq and Syria, yet it’s succeeding beyond its hopes in Europe. It’s doing so because there are people, usually young men of Muslim background, who fall in love with violence, death and IS. It has plugged into a hellishly rich vein of youths who feel that life has nothing better to offer them than glorious murder and a martyr’s death.

IS acts within what a French radical, Guy Debord, called “The Society of the Spectacle,” in which representations are everything, a state of affairs which can only be combated “through radical action in the form of the construction of situations.”

The situations that IS devotees construct smash through what they regard as the banal images and activities of daily life, to prepare for the victory of a pure Islamist society.

The crowning irony of the quest is that the jihadis use radical post-modernism to haul the democratic world back into an authoritarian medieval inferno. In Iraqi cities controlled by IS, from which some residents managed to escape, women were confined to their homes and covered from top to toe. Among other disciplinary practices, men caught smoking had a bar of red-hot metal pushed into their mouths.

Meanwhile, the strategy to destabilize liberal societies is working. New bills are progressing though legislatures in France, Germany and Britain. France’s state of emergency has been extended. Churches may soon follow the example of many synagogues and hire guards, or be given police or army protection.

The philosopher and activist Bernard-Henri Levy has recommended that Western societies emulate the citizens of Israel, and develop a “sixth sense” to detect impending danger. Israel is the target of Hamas and Hezbollah terrorism, and its neighbors are confronting serious civil strife.

That a leading French public intellectual is seriously recommending that Europeans and North Americans adopt an Israeli mentality illuminates how IS has “framed” us.

John Lloyd co-founded the Reuters Institute for the Study of Journalism at the University of Oxford, where he is a senior research fellow.

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