Old secularization theories die hard. The twenty-first century was supposed to be a new age of enlightenment in which progressive politics and bourgeois economics domesticated religion, family, and bonds of national kinship, making us morally unencumbered global citizens. Then came ISIS, Boko Haram, and other representatives of the vicious new wave of Islamic extremism. More surprising than the brutality and violence of their crimes is the way these groups have infiltrated our society. Why do young people who should be enjoying the fruits of liberal democracy become radicalized? Much of our intellectual elite does not see religion as a significant factor in human society and history. For them, the answer to this question must be economic or social – hence President Obama’s persistent assertion that, despite its own claims to the contrary, ISIS is not an Islamic or theological problem.

But as the violence mounts and as men and women who are neither poor nor culturally marginalized commit mass atrocities, the heads have begun to come out of the sand. In one of the most read and debated articles of the past year, Graeme Wood argued in The Atlantic that ISIS and Boko Haram are a theological problem (“What ISIS Really Wants,” March 2015). Whatever the influence of economics or other factors, their followers are driven by a particular theology with a particular – albeit ­minority – pedigree and roots in Islam. Wood is right: Islamic extremism is a theological problem. But how do we go about solving it? The solution to the theological problem must be theological, not military. And the best proposal to date comes from Rabbi Lord Jonathan Sacks, former chief rabbi of the United Kingdom.

Not in God’s Name is primarily a work of ancient biblical interpretation, but it begins with contemporary social theory. Sacks calls evil committed in the name of a sacred cause “altruistic evil,” the kind that turns “ordinary non-psychopathic people into cold-blooded murderers.” They slaughter with delight, confident that they are doing God’s work. The problem is not religion, per se; as the twentieth century’s body count from totalitarian regimes makes clear, substitutes for religion lead to more violence. Rather, it comes from two factors: our “groupishness” – the fact that we are dependent, relational animals – and dualism. Our social nature leads us not only to altruism (usually directed toward members of our group), but also to suspicion and aggression toward outsiders.

Marc Chagall, The Sacrifice of Isaac, Musée National Marc Chagall, Nice, France.

Religion is implicated as the most powerful source of group identity many have. And religion offers a way to explain why the world is not as it should be. If these explanations retain a complexity matching the world’s, they can be healthy. The problem arises, Sacks argues, when they become pathologically dualistic – when the evil in the world is monochromatically attributed to “them.” This kind of dualism dehumanizes opponents and allows believers to see themselves always as victims, irrespective of evidence to the contrary.

What is it that makes dualism pathological? Here Sacks borrows from the French theorist René Girard (1923–2015), who argued that societies begin with murder and violence between two figures or groups. The most effective way to resolve that violence, according to Girard, is to blame it on a third, the scape­goat. Sacrificing the outsider allows both sides to feel that justice has been done, but not at the expense of either group. Religion performs the important task of casting out and ending the violence that would otherwise rend society asunder. But Girard’s theory has a second part. The violence comes in the first place from mimetic desire: desire that imitates the desire of another (think of Cain and Abel). Sibling rivalry – the desire to have what your brother has and be what he is – lies at the heart of our conflicts.