After terrorists killed at least seven people in London on Saturday night, in the third attack in three months, Theresa May declared that “Enough is enough.” No right-thinking person would disagree with the Prime Minister. The problem is that there is no consensus on how to prevent young Muslim men raised in Britain from becoming mass murderers.

In France, the debate about this new form of terrorism is further advanced. For the past few years two intellectuals have duelled over the causes of jihadism in Europe. The first, Gilles Kepel, is a professor of Islamic studies at the prestigious Sciences Po. The second, Olivier Roy, is an ex-Maoist political scientist teaching at the European University Institute in Florence.

In Terreur dans L’Hexagone (“Terror in France”), Kepel argues that the root cause is the spread of more aggressive forms of Islam. These strains find fertile ground in France’s economically stagnant and socially isolated banlieues, and breed small armies of jihadists to fight at home and abroad. What we’re seeing, in other words, is the radicalisation of Islam.

On the contrary, says Roy, we are actually witnessing the “Islamisation of radicalism”. Europe’s new terrorists aren’t motivated primarily by economics, history or even religion, he suggests in Death and Jihad. They are mainly second-generation immigrants, caught between the insular, tradition-bound communities of their parents and the unconstrained freedoms of their secular contemporaries. Finding themselves adrift, they opt for nihilistic rebellion. If they lived in the 1970s, they might have joined violent Marxist movements. Instead, they opt for the 21st-century equivalent: Islamic extremism. For Roy, Islam is simply a vehicle for psychologically disturbed young men to act out their violent fantasies: a bloodthirsty youth movement.

The debate between Kepel and Roy quickly became highly personalised and was, of course, inconclusive. As an Arabic speaker with a deep knowledge of Muslim history, Kepel has the scholarly edge. But it’s striking how many jihadists fit Roy’s psychological profile. This clash is not merely academic: how we understand this grave new phenomenon will determine how we fight it.

Catholics will see another factor at work in the new terrorism, beyond history, psychology, sociology and economics: the power of darkness. The Church has long taught that Christians must engage in spiritual warfare: resolutely resisting evil both within ourselves and in the external world. St John Paul II, who saw the horrors of the 20th century at close quarters, often spoke of the mysterium iniquitatis, the puzzle of why sin still abounds after Christ’s triumph on the Cross. He argued that the best way to oppose this mystery, which can’t be grasped by reason alone, is to embrace another one: the mysterium pietatis, or “mystery of piety”, meaning the whole of Catholic teaching and practice.

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