I can't speak for others but certainly my experience is that to be a Christian you do have to believe in God. I strongly contest that you have to engage in any traditional rituals in order to be a christian though. My understanding is that being a Christian means believing in God and living your life by following Christian values but all the other stuff to my mind is optional. My father has in the past attended the Community church in Southampton- a "church" pretty far removed from my idea of a church at any rate. I don't know about now but when I attended in the past there was no symbolism as such. There were songs sure but it was up to the individual how much they participated or not (which IMO is how it should be!). I respect the faith my father has though I do not share it to the same degree precisely because the last thing he ever does is ram it down people's throats. His belief is that he shows people he is a Christian through the way he lives his life so there is never any need to go out of his way to mention it.
I think there would be some disagreements about your statement that religions believe in the same God. My experience is that believers of religions believe in one God but that it is not necessarily the same. In regards to your last point, I would say that the belief in God is pretty fundamental to being a Christian (though of course it is entirely possible to live by the teachings of Christianity though not believe in God.)
Thanks for the interesting conversation, I of course can't even begin to know any of the answers but that's how I see it (just to make it blindingly obvious to notnowcato.)