Historian Tom Holland is walking away from it all, albeit temporarily. Aware that his new documentary, Isis; Origins of Violence, would divide opinion, he booked a walking holiday in the English countryside.

“I thought, I’ll take the blowback and then get away from it all, but you tracked me down, as of course did the news from Manchester.”

He sounds tired, not just from the day’s hiking, but with the knowledge of the Manchester terror attack.

“You carry it with you — the idea that young girls can be targeted because they are young girls at a concert. I find it more upsetting than perhaps any other attack in Europe”.

Holland knows the violent Islamist rationale behind such barbarity — that any non-believer from the “Crusader” countries is a fair target — because he has been studying the ideology of Islamist terror groups for years.

Speaking to the Evening Standard before the attack, he argued that Isis “is pretty Islamic to the degree that they consider themselves to be Islamic

“The mistake people make is to replicate Isis’s position, which is that there’s one, true form of Islam and anyone who deviates from that isn’t a Muslim. That’s Isis’s justification for killing Shia Muslims.

“Ironically, when Western leaders say ‘it’s nothing to do with Islam’, they’re doing the same. I don’t think it’s the business — particularly of non-Muslims — to specify what a Muslim is. If people say they’re Muslim, they’re Muslim… The vast majority of Muslims are appalled by what Isis does. But if they’re quoting chapter and verse from the Koran, there needs to be a firewall built between the normative practice of Islam and what Isis is doing.”

When Origins of Violence was broadcast there was instant praise and criticism. The film was described as brave by its supporters but elsewhere attacked as pro-Isis propaganda because it sought to explain that the Isis worldview is rooted in its interpretations of the Koran.

Nevertheless, the reaction was less violent than to his previous documentary Origins of Islam, broadcast five years ago.

“Then, the reaction was really horrible — multiple death threats,” he says. “Within an hour, the Met were onto me asking about security so I was slightly nervous. This time there have been a few responses but not like before. Why? I think Origins of Islam was a shock to many people because I challenged the origins of the Koran and that very possibly came as news to a lot of people. A lot of Muslims thought I was either an idiot or in the pay of Mossad, or both. But this time with Origins of Violence, it was saying something which lots of people are aware of.”

So, does he think these origins need a “Reformation”?

“I think that formulation is terrible. It hovers behind the protestant Reformation. It assumes that religions follow the same paths, but that’s ridiculous, Islam has a very different cultural DNA to Christianity. By and large we view things through the Christian prism, and Christianity does not provide a template on which to judge other beliefs.”

Perhaps “reform” is a better word?

“Well, by reform what people mean is making it compatible with western liberal democracy — they mean a westernised Islam. But I make the point in the film that Islam has been radically westernised in the past 200 years. Abolition of slavery, and the ending of the Jizyah, (tax on non-Muslims) came about partially because of contact with the West.”

Isis cast themselves as the real reformers of Islam, those who want to cleanse Islam of this Western influence. So, who does the historian think is winning the argument?

“Globally? I don’t know. In Egypt, we spoke with a journalist who is a Muslim who said the clash was between Muslims who believe they are superior to non-Muslims and Muslims who don’t think that. He wasn’t sure who was winning. In the UK I think the huge majority of Muslims are on the reforming side, but as we’ve seen in Manchester, not all.”

Tim Marshall is author of ‘Worth Dying For; The Power and Politics of Flags,’ published by Elliott and Thompson