How Islamic is Islamic State? This question has prompted much agonised equivocation from politicians, broadcasters and pundits. Implicit in the BBC’s dogged insistence on referring to IS as “the so-called Islamic State” is a desperate yearning to believe that it has nothing to do with Islam at all. How much more reassuring it is to blame the ­organisation’s crimes – not to mention its appeal to Muslims who have travelled from across the world to join it – on Western ­foreign policy, or anomie, or perhaps a lack of sex. Anything, in short, rather than contemplate the possibility that the ­murderous savagery of the jihadis might indeed be fuelled by an authentically Islamic ideology.

Such a conceit has become a good deal harder to sustain, however, with the publication of Shiraz Maher’s groundbreaking study of what he terms “Salafi-jihadism”. A senior research fellow at King’s College London and a contributing writer for the New Statesman, Maher combines scholarly objectivity with something no less valuable for someone trying to make sense of Islamism: personal experience of campaigning for a caliphate. Radicalised after the 11 September 2001 attacks, he spent four years rising through the ranks of Hizb ut-Tahrir, a Sunni organisation committed to establishing a global Islamic state, but repudiated it on the day of the London Tube bombings. When Maher delivers an evaluation of Salafi-­jihadism, he does so with the confidence of a man who knows whereof he speaks.

So, it is striking that he should open his book by slapping down his cards very firmly on the table. “Yes,” he ­acknowledges, “Islamic State is more brazen and ruthless than its predecessors but the ideas that guide it are well established in radical Sunni thought. Their roots are grounded in the experiences of Sunni Islam over the last century and beyond.” Maher’s ambition in his book is to justify this assertion, and to explicate the confluence of doctrines and ideals that, bred of old Islamic thinking, have combined over the past three decades to such novel and literally explosive effect. The result is a masterclass in how to do intellectual history, and one that nobody with an interest in radical Islam should miss.

“Salafism is a philosophy that believes in progression through regression.” The impulse that lies behind this striking formulation derives from the fundamentals of how Islam has historically conceived of itself. According to Muslim orthodoxy, there was only ever the one truly golden age. It was those who had been with the Prophet, as he delivered both the Quran and his rulings on what it was to be a good Muslim, who provided the surest model to the faithful. “For at that time,” as the Arab polymath al-Jahiz noted in a wistful tone two centuries after Muhammad’s death, “there was nothing in the way of offending action or scandalous innovation, no act of disobedience, envy, rancour or rivalry.” As such, an obvious recourse for those anxious to practise a pristine form of Islam, purified of all accretions and distortions, has always been to look back to the first three generations of Muslims: the “Ancestors”, or Salaf.

In that sense, Salafism has a venerable pedigree. Maher traces it back to Ibn Taymiyya, a medieval scholar whom radicals cherish today as “the shaykh of Islam”, and to al-Wahhab, a preacher from Nejd, in central Arabia, who in the 18th century inspired a reformist movement of such austere and enduring potency that “Wahhabism” is often used today – though most certainly not by Salafists themselves – as a synonym for Salafism. Nevertheless, just as Luther’s attempt to restore the Church to its primal condition helped to unleash the Protestant Reformation, so has ­Muslim reformers’ obsession in recent decades with returning Islam to its roots served to trigger its own revolutionary ferment. Maher’s book, with its cast of obsessional ideologues arguing furiously and often violently over details of scripture, all the while condemning co-religionists of whom they disapprove as idolators and waiting eagerly for the end of the world, will seem eerily familiar to students of Anabaptist Münster or Calvin’s Geneva. All those calling for an Islamic reformation should beware what they wish for. Maher’s book prompts the disquieting reflection that perhaps it has already arrived.

Why precisely the pace of this metastasis should have quickened so noticeably over the past few decades is a question that haunts Salafi-jihadism. Bred of the marrow of Islamic scripture though it may be, those who would identify the impact of the West as well on what has been happening are not wrong. The lethal doctrine of takfir, which has repeatedly enabled jihadis to justify the murder of fellow Muslims as apostates, and which they trace back to the intimidating authority of Ibn Taymiyya, was most dramatically pronounced in 1981 with the murder of the Egyptian president who had made peace with Israel, Anwar al-Sadat. The decade that preceded the assassination had been a time

of mounting anxiety on the part of many Egyptian Muslims that their country had become something sinister and hybrid: outwardly Muslim but infected by disbelief. Those who had permitted this to happen, it was argued, richly merited death. This conviction, over the decades since Sadat’s assassination and his murderer’s triumphant assertion that he had “killed Pharaoh”, has claimed countless Muslim lives. The motivation is always the same: to scour dar al-Islam of what is perceived as the baneful taint of the West, and to render it pristinely and uncompromisingly Islamic, as it is supposed to have been.

It seems no coincidence that the growth of jihadism, viewed in such a light, should so closely have paralleled the course of globalisation. The more that Muslims have come into contact with people of differing backgrounds and beliefs, the more some of them have come to fret that their religion is ­being diluted. Ayman al-Zawahiri, Osama Bin Laden’s heir as the leader of al-Qaeda, is not alone in mourning the world’s increasingly multicultural state. “Previously, Muslims resided in the realm of Islam and the infidels in the realm of war . . . Nowadays there is no such thing, and the people are mixed together.”

Al-Qaeda and its ideologues, looking for a solution to this crisis, naturally turned to the great corpus of Islamic scripture, which they interpreted as they believed the Salaf would have done. The consequence – by a familiar paradox of religious history – was to set them on a revolutionary course: for their reading of Allah’s purpose brought to the poetry and mystery of His revelations the literalism of the engineering manual.

“The violence of groups like al-Qaeda and associated movements is neither irrational nor whimsical,” Maher writes. His study of their motivation, detailed and definitive as it is, leaves no room for doubt about this. He writes as a scholar – and a work of scholarship is what he duly gives us. Nevertheless, it leaves hanging an unsettling question. If Islamic State is indeed to be reckoned Islamic, then how are the many Muslims, the vast majority of the faithful who recoil from their actions, to quarantine themselves and their beliefs? To that, Maher gives no answer. I hope, though, that he is working on it – and that it will not be long in coming.

Tom Holland’s books include “In the Shadow of the Sword” and “Dynasty: the Rise and Fall of the House of Caesar” (both Abacus)

Salafi-Jihadism: the History of an Idea by Shiraz Maher is published by C Hurst & Co (296pp, £25)