Usama Hasan was the imam of a mosque in Leyton until he was driven from the post by death threats. This is perhaps the most extreme reaction there has ever been to an article on Comment is free: the death threats were the response of a section of his congregation to a piece he wrote here defending the truth of evolution. He kept his head down for a couple of years after that, to protect his family, but has now resurfaced as a fellow at the Quilliam Foundation, the counter-extremism thinktank.

At the weekend, he was in Salisbury, at the Muslim Institute's Winter Gathering, and I chaired a discussion with him there on creationism among Muslims. In close-up his story was even more shocking than it appears in summary. A visiting Saudi cleric issued a fatwa, from the Green Lane mosque in Birmingham, that supported his enemies in the congregation: not only did it explain that anyone who denied creationism was an apostate, who could (and should, in an ideal state) be killed, but that his support for women going bareheaded if they wished, and for a secular form of government, were also sufficient grounds for a death sentence.

Since these judgments were circulated in jihadi circles, Hasan and his family were in real danger as a result and were granted police protection for a while.

This is shocking enough as an example of what Saudi influence on British Islam can lead to. I'd like to believe that visiting preachers are now rather more careful about what they say in public. But in the absence of hard evidence either way I will maintain an attitude of suspicion. Wahhabi Islam really is a loathsome and dangerous ideology.

We knew that, already, of course. It's good to be reminded. But there are some small aspects to his story that deserve consideration. I asked him which of the three "crimes" he had been charged with was the most serious. It's not easy for the western mind to understand how they fit together. To us, evolution is a scientific theory, the headscarf is a matter of fashion, and secularism is a political programme. How can any of them – let alone all of them – be considered so dangerous that their proponents must be killed? Which was the real offence?

He replied that they were all equally serious, except that to some people the headscarf mattered less – to others it was the most important. They all appear to fanatics as unpardonable violations of a pure Muslim identity.

In this country, of course, secularism is unavoidable, and so is the knowledge that many Muslim women choose not to wear headscarves, and many more the full veil. But creationism can remain an invisible marker of Muslim identity and there's some reason to suppose that it remains so.

Nearly half the audience in Salisbury had been raised as creationists, and they are a core part of the intellectual elite of Islam in this country. Many in fact worked as scientists. But it emerged clearly that you can be a creationist without it affecting any other part of your life. Hasan himself took a degree in theoretical physics at Cambridge and later a doctorate in artificial intelligence – but remained a creationist through all those years. He admitted that his 20-year-old self might well have called for the death of his 40-year-old self.

So what can be done about this? In the short term, it is obvious that outrage is quite useful. There are some things – death threats are a good example – to which the answer is "You just bloody can't do that in a civilised country". In the long term, we must hope for social change which involves hundreds of thousands of individual changes of heart. And that is where the limitations of outrage become apparent.

Contempt is a hugely counterproductive tactic when it comes to creationism. Sure, it makes the contemptuous feel good. But since creationism is adopted and transmitted as a marker of identity, contempt becomes aimed not at the idea (which is most unlikely to be understood, even assuming it is coherent) but at the identity which it expresses. That really doesn't work on a minority that already feels excluded and discriminated against. Assimilation is based on the assumption that the mainstream culture is superior – as it clearly is superior to Saudi Islam – but the paradox is that it can only work when it involves genuine respect for the cultures that are to be assimilated.