"If you can say that sincerely, you're a Muslim. If you can't, you aren't. Muslims argue about a lot of things, but they don't get into arguments about who is a Muslim and who isn't."

I don't think that holds water though. I can recall talk show after talk show where Moslems members in the audience were denying that the London bombers were Moslems - yet the bombers themselves believed they were acting in the name of the said religion. Similar frictions within the religion! Likewise they split into factions within the religion itself. There is much sectarian conflict in certain countries between Sunnis and Shia etc.