The Christchurch massacre of Friday worshippers in two mosques on March 15 focuses attention as never before on festering racial hatreds and the rising tide of violence that gives expression to them. This small New Zealand city was considered a most unlikely location for mass murder, especially targeting Muslims. New Zealand, unlike Australia and other white-dominated societies further west, is well integrated with its non-white population and a stranger to the kind of systematic carnage let loose by 28-year-old Brenton Tarrant.

The Australia-born suspect not only mowed down people at random in the Al-Noor and Linwood mosques but also live-streamed it on Facebook after posting a 73-page online manifesto titled “The Great Replacement”. That makes him not just a mass murderer but an ideologue as well, on the lines of Islamic State fighters or the Taliban of Afghanistan. The manifesto shows him to be a white supremacist but the obsessions and fixations they articulate share a remarkable similarity with the jihadi mindset.

The concern with racial purity is almost a verbatim description of the corruptions that have fatally compromised the practice of Islam, along with fears of the liberal order consigning the white race and Islam to undeserved obscurity. Both have their histories and myths of ancient power and glory, both dream of restoring lost dominions and share nightmares of imminent obsolescence and irrelevance. But these neuroses are not confined to white supremacists or Islamists.

Bible-thumping Christians and Hindutva devotees have their own demons to exorcise. With the first it is the Antichrist, anything or anyone not Christian or not in tune with their particular brand of faith, or atheists who prefer the evidence of science to the certitude of true belief. For the second, it is the Muslim invader who destroyed a land of milk and honey and dishonoured its gods. Centuries of humiliation remain unaddressed. Support for the saffron brotherhood hinges precisely on the extent to which this promise is honoured.

Whether these are unhinged fantasies or myths wrapped around a kernel of truth the point is that a great many people subscribe to them, lock, stock and barrel. That seems to be the final test of truth for a lot of people. Tarrant’s assault weapons were covered with the names of historical events and people connected to conflicts between Muslims and European Christians, names of IS victims and far right figures. Presumably they were all taking part in his crusade.

We are only now realising that fact-free belief is much more prevalent than we thought. Neither the advantages of education nor the explosion of accurate information online and the many ways of delivering it seem to be having much impact. In fact, the believers are exploiting the new media to amplify their messages. Christians use laptops and audiovisual aids to put out their homilies against abortion, homosexuality and promoting Creationism. That the philosophy underlying all this technology is an atheistic insistence on the laws of nature not God is an irony that passes them entirely.

In these readings, the US civil war was not against slavery but to preserve the southern way of life, India’s national movement was not only about ending British rule but also a reassertion of the Hindu way, the white supremacist movement merely seeks to restore the natural law of racial superiority, and so on. The painstaking rational fact-finding that undercuts each thesis might as well not exist for the believers.

“What I tell you three times is true,” the Bellman says in The Hunting of the Snark. The internet repeats it a billion times and helps the pilgrim find ten thousand kindred souls in every corner of its domain. The chorus and harmonies set up by the massed ranks of believers make any “objective” definition of truth almost irrelevant. The ever-increasing amplification of even the most obvious paranoia eventually becomes its own weird validity.

We are creatures of emotion and instinctual response rather than reason, especially when faced with what we think is an existential moment. That explains some of our bigotries and lapses from civilised behaviour. But the worst instances, like 9/11, Godhra and aftermath, Mumbai 26/11, or Bataclan, Paris, 2015, are the result of systematically exploiting divisions. This is where leadership makes a difference. Compare the shifty evasions of Theresa May with the solemn decisiveness of Jacinda Ardern. One led to the serial satire that is Brexit, the other has made Christchurch a teachable moment.