I have been re-reading the Koran recently. It's a dispiriting experience for an unbeliever, especially one who comes to it with an open mind. No matter what good works I may do in this life, my disbelief is sufficient for the "curse of God" to be upon me. An "evil fate", a "lasting torment" awaits. Regardless of my actions, I am condemned as one of the "heirs of Hell", fuel for Allah's everlasting fire.

It's dispiriting because I'd hoped that by going back to the source I'd find something different, more inclusive. I'd hoped to find something closer to the philosophy espoused by my Muslim friends - one that judges people more on what they do than on what name they give to God in their prayers. I'd wanted to be able to blame the flaws in fundamentalist Islamism on the followers, not on Allah.

In fact, the more I read of the Koran, the more it seems clear that it is Allah's followers (or the non-fundamentalist ones amongst them, at any rate) who put a beneficent spin on his revelations - and it is Allah himself whose words are deeply flawed. If every letter in the Koran is Allah's infallible own, as believers insist, there's no room for argument, no scope for interpretation. It's not someone else but Allah in person who must take the rap.

So, for example, it's not the Islamist extremists distorting a message of peace when they say that "Al-Fitnah [disbelief, idolatry] is worse than killing." It's the Koran, the perfect expression of Allah's will. And it says so more than once, just to make sure there's no misunderstanding. Disbelief is worse than killing: truly words to chill you to the bone.

Sadly, the mere act of making these statements means that I am cutting myself off from dialogue with the more rigid "true believers" of Islam. The Koran does not permit dialogue with those who do not believe. It enjoins Muslims, at best, not to waste their breath on the likes of me or, at worst, to fight and kill me.

Not even friendship with non-believers is permitted for Muslims if they're going to abide strictly by Allah's injunctions:

"Believers, take not as bitanah [advisors, consultants, friends, helpers] any but your own people." (3:118)

Or, more explicitly: "Believers, take neither Jews nor Christians for your friends." (5:51)

Some British Islamists have been telling people arrested under anti-terrorism legislation, or subjected to racist or anti-Islamic treatment, that injunctions such as these prohibit them from accepting help from non-Muslim supporters. It is forbidden, so this argument goes, for good Muslims to use non-Muslim solicitors or even to seek redress under a non-Muslim legal system. One sheikh is cited saying that, "One of the factors that turns a Muslim into an apostate [for which the penalty specified in the Koran is death] is seeking any judgement other than that of Allah and his Prophet."

Are these the arguments of a warped extremism or a literal reading of Koranic texts? The answer, unfortunately, is both. It is difficult, in any honest reading of the Koran, to avoid the conclusion that it contains within itself the fundamental rationale for such extremism.

The Koran is no different to the bible in this respect, and it would be wrong, even in a short column such as this, to pass over the fact that there is much that is wise and good and merciful in its contents. Again and again, it makes clear that belief is not enough: it must be accompanied by "good works" in order for the faithful to ascend to paradise. The giving of alms (crucially, for the glory of God and not to earn praise for oneself) is central to Islam in a way that would make the average western capitalist blanch. Not for nothing does Islam have such a powerful appeal to the poor and powerless.

But the Koran still carries what is, in essence, an exclusive and intolerant message. The rewards of paradise are reserved for those who believe in every last jot and tittle of that message; and who, literally, submit to the God who supplied it. As for the rest of us, "They are the heirs of the Fire, and there they shall remain for ever."

If that was all the Koran had to say on the subject, it might be possible to agree to differ and let those of us who are unbelievers take our chances with the rest of humanity on the Day of Judgement. It's not, though, and therein lies a terrible dilemma for every non-believer who wants to stand firm with Muslims against racism and discrimination and in support of religious freedom.

How do we square the circle of standing steadfast in defence of religious tolerance, when so many of the world's religions (Islam is far from being alone in this respect) would deny that tolerance to others?