isn't it the case that Buddhism is not so much a religion, but a philosophy? As you point out it is not concerned with the soul, how we got here, where we are going to, realities outside our own, God etc - i.e. the tradional remits of religion.
As such you can be a Buddhist Christian, a Buddhist atheist, a Buddhist agnostic, or a Buddhist whatever for that matter.
PS. My favourite Zen story:
"A student was one day visiting the grand master, to whom he was only allowed to ask one question. In his eager quest for knowledge he was thinking for the perfect question.
He asked the master "What is the best question I could ask you, and what is the best answer to that question you could give?".
After a while pondering the master replied:
"The best question you could have asked is the question you have just asked, and the best answer I could have given is the one I have just given".