Mr Tea - I think Islam is better understood as an "ethnic identity" than a belief system, as are many other religions as practised.
Oh come off it Dan, it's a religion and a religion is a set of beliefs. You're not much of a Muslim if you don't believe there is one (and no more than one) God, that Mohammed was his final and definitive prophet and that you ought to give the beers and bacon sarnies a wide berth. And yes of course there's no hard line between ethnic identity, culture and religion, but I don't know if (say) Arab Muslims, Indonesian Muslims, black African Muslims and white European Muslims necessarily recognize each other as belonging to the same "ethnic identity", any more than people in Armenia, Uganda, Texas and Venezuela would, just because they're Christians. Co-religionists, sure, but that's not the same thing.
Also I find it perhaps a bit disturbing to see religion becoming an "ethnicity". I mean, someone born white or black or an Arab will always belong to that ethnic or racial identity, but someone who is
brought up a Muslim, Christian or whatever can choose to convert to another religion or indeed abandon religion altogether. It's not fixed at a genetic level in the way facial features and skin colour are. It's not even really comparable to language, which is probably the other most important signifier of identity, because a) with the exception of people with a severe mental deficiency, everyone on earth can speak at least one language, whereas plenty of people get along fine without a religion; b) it's perfectly possible to speak two or more languages, whereas most of the major religions are mutually exclusive (it's hard to see how anyone could in any meaningful sense be both a Jew and a Hindu simultaneously - though I concede it's a bit different with certain East Asian religions), and c) learning a new language takes a long time and a lot of effort, whereas some religions can be adopted almost on a whim or indeed forced on someone by threat of violence.
Further, religion-as-ethnicity feeds the idea that to criticize any aspect of a religion practiced mainly by people who don't look like you is "racist", which just ends up stifling debate and privileging belief over other important aspects of human rights and identity.