On 6 July 1535, the severed head of England’s former lord chancellor, Sir Thomas More, was carried across London Bridge to the gatehouse on the southern bank. There it was parboiled and set on a spike. Another head, that of the bishop and theologian John Fisher, was removed to make way for it, and thrown into the Thames. Both men, rather than accept Henry VIII as Supreme Head of the Church of England, had willingly embraced martyrdom at the king’s hands. Both men would end up canonised by the Catholic Church. Amid the violent convulsions of the Reformation, nowhere bore more public witness to the willingness of men to kill and be killed in the cause of God than London Bridge.

Time, though, would see the intensity of these religious passions fade. What a cooler and more whiggish age would dismiss as ‘enthusiasm’ passed out of fashion. By 1974, when a tourist attraction named the London Dungeon opened on Tooley Street, just south of where More’s head had once been exhibited, religious fanaticism appeared as much a part of Britain’s past as public hangings or the Black Death. The causes which had led a succession of English monarchs during the Reformation to torture and kill those they condemned as heretics appeared so drained of contemporary relevance that they had become the stuff of jokey waxworks.

But then, last Saturday night, religiously motivated killing returned to London Bridge. Three men, swerving to murder as many pedestrians as they could, drove a rented van across the very spot where severed heads had been fixed to the bridge’s southern gatepost. They crashed opposite Tooley Street. Then, brandishing long knives, they plunged into the warren of streets and passageways around Southwark Cathedral where, back in the reign of Mary, six high-ranking clergymen had been tried and convicted of heresy. For eight terrible minutes, terrorists — no less convinced than Tudor inquisitors had been that they were the agents of a stern and implacable god — visited slaughter upon Borough Market. Just four days later, another group of Islamists, equally fanatical and set on martyrdom attacked the Iranian parliament and Ayatollah Khomeini’s mausoleum in Tehran, killing at least 12 people and injuring many more.

The London Bridge attackers wanted us to be in no doubt about their motivation. ‘This is for Allah,’ they shouted, as they slashed and stabbed their victims. When they could, they slit people’s throats — just as Isis executioners in Syria, claiming obedience to a command in the Quran ‘to strike off the heads of unbelievers’, had slit the throats of western hostages. Shot by police marksmen, the three men were hailed by supporters of Isis as ‘martyrs’.

Sometimes it can be hard to recognise ghosts for what they are. Reactions to the atrocities committed on Saturday — as to the atrocities committed only a few short weeks previously in Manchester and on Westminster Bridge — have mingled despair with perplexity. We just don’t understand violent religion.

The Church of England, offspring of the Reformation though it is, has long since forsworn the sugar-rush of fanaticism. Vicars today rarely quote from Foxe’s Book of Martyrs. Christianity in Britain has made its peace with the values of the Enlightenment: secularism, liberalism, humanism. The ease with which it has done so reflects the degree to which, in large part, it came to serve as their midwife. The God who provided assurance to Tudor monarchs that they were justified in killing heretics, and to Cromwell’s major-generals that England should be ruled as a theocracy, is today far more likely to be found in the nightmares of Richard Dawkins than in the prayers of Christians. The Church of England these days is far too busy wringing its hands over its faults and failings to demand vengeance on those of others.

Not that Anglicans are alone in their commitment to what is, theologically speaking, milky tea. Whether in GCSE Religious Studies lessons, or the speeches of politicians, or the manifestos of diversity programmes, all religions are presumed, in their essentials, to be pretty much the same. Every speaker who appears on ‘Thought for the Day’ — Catholic, Muslim, Hindu, it makes no difference — subscribes to the same inoffensive tenets. God is peaceful; God is politically correct; God probably votes Liberal Democrat. Yet while these are undoubtedly assumptions that have served to oil the wheels of multi-cultural Britain, and enabled most people to maintain an attitude towards religion of mild and relaxed indifference, their promotion is not necessarily, perhaps, as tolerant as it might at first sight seem.

Their guiding principles — that religion should properly rank as a matter of personal belief, that church and state should be kept distinct, and that the law of the land should trump any legal system claimed to derive from a god — are not nearly as universal as most people in Britain might like to think. Shaped by the twin inheritances of Christianity and the Enlightenment, they are in reality decidedly contingent. Indeed, to assume that all other belief systems, all other ways of ordering existence, can simply be folded without difficulty into the liberal embrace of our secular state risks verging on the complacent — or the arrogant.

When, for instance, in her speech after the London Bridge attacks, Theresa May decried as a perversion of Islam any notion that it might be incompatible with Western concepts of human rights, she had things precisely the wrong way round. Classically, Muslims have believed that no human rights exist except for those that have been prescribed by God. In the Quran, it is taken for granted that the only way to know how to live well is by means of a divine revelation. Naturally, the Quran casts itself as the ultimate source of these revelations.Anyone who does not submit to the god revealed in its pages is, then, by definition, inferior to those who do. This is all the more so because, according to one of Mohammed’s most quoted sayings, every newborn is naturally Muslim. ‘Then his two parents make him a Jew, a Christian, a Zoroastrian.’ Non Muslims, in other words, are effectively apostates. This is why, should they subsequently convert to Islam, they are termed by many Muslims ‘reverts’. It is also why, for a millennium and more, Islamic states actively discriminated against them. A Quranic injunction decreed that Jews and Christians pay a humiliating tax, the jizya, in exchange for the right to practise their respective religions. Those defined as mushrikun — ‘idolaters’ — were denied the right to practise their religion at all.

Does this mean, then, that Islam is inherently a supremacist religion? Not at all. Like Christianity, it has experienced a long weathering by the Enlightenment; and many Muslims today, like many Christians, have not the slightest problem in living by its values. Over the course of the 19th and 20th centuries, across the Islamic world, the jizya and the slave trade went the way of the Spanish Inquisition; today, in multicultural Britain, the vast majority of Muslims would have no truck with the notion that they are superior to non-Muslims by virtue of being Muslim.

And yet, for all that, it is clear that the legacy of Islamic supremacism, deriving as it does from both the Quran and sayings of Mohammed, still has a potent and seductive appeal. Indeed, there is a sense in which it may be precisely the integration into Islam of the Western notion of human rights that is helping to fuel its recrudescence. After all, if — as Muslims believe — their religion is the last and ultimate of God’s revelations, then any dimunition of its purity, any dilution of its traditions, can all too easily be portrayed as a lethal threat to the entire future of humanity. Isis, who have pointedly reintroduced both the jizya and slavery, are merely the most extreme of those factions within Islam who insist that Muslims, far from compromising with the values of the West, should instead seek to destroy them utterly.

We are witnessing a civil war within Islam and the three men who brought

carnage to Borough Market last Saturday did not see themselves as murderers, but rather as warriors. They imagined that they had been divinely summoned — just as Mohammed had been — to the overthrow of kufr: unbelief.

No laws, no increase in police numbers, no boost to the powers of the security services can adequately patrol such ideas. Only by directly confronting these beliefs do we have even the faintest prospect of diminishing their potency. To do that, though, will first require acknowledging what Isis and their cohorts in the West actually embody: a strain of Islam that has its roots deep in the past, and which, as our most careful analyst of Isis, Shiraz Maher, has put it, ‘believes in progression through regression’. To dismiss it, as Theresa May did, as ‘a perversion of Islam’ is not merely to close our eyes to the nature of the threat that it presents to Britain’s future as a free society; it actively risks making it worse.

So as we begin the inevitable discussion about what to do next, the first step ought to be a fairly basic one: recognise the problem.

To assume all belief systems can be folded into the liberal embrace of our secular state verges on the arrogant

Tom Holland, a historian of early Islam, is a dinosaur enthusiast and a translator of Herodotus’s Histories. Spectator.co.uk/podcast Tom Holland and Christopher de Bellaigue on Islam and the modern world.