I agree with you that there is only one God. I believe God has been interpreted many ways but all of these interpretations somehow reduce to only one existence. The question is how much lenience do we have in our religious interpretations of God such that these interpretations are any more accurate than a comic book. For example, I've heard that recovering alcoholics will benefit from a belief in God, but 'God' for them might be anything. Yes, this view may have some relationship to God as an existence, but it is heavily affected by human interpretation and imagination.
Einstein was a theist, but he rejected personal theism and especially detested anthromorphisms (God has human appearance) and anthropathisms (God has human conduct). However, if God is completely non-anthropathic, then he either doesn't exist or his existence is a complete mystery. Religion is that aspect of human endeavor which seeks the anthropathisms of God (i.e., the human ways of viewing God). The more anthropathic versions will make God into a thunder God, or a warrior God, etc. These anthopathic versions are out of vogue even by most fundamentalist conceptions (although not all).
What is not out of vogue is to view God with human characteristics of emotion, intelligence, strength, etc. I don't know how much we are willing as a religious society to depart with some of those anthropathisms. My view is that we should be prepared to depart with more of these and start looking upon God as a Mystery of mysteries. There is one God, but it is not like having one pencil, one computer, one paper pad, etc. Rather, having one God means to have one kind of existence that simply is definable more by what God is not then what God is. Of course, this is only my inclination...
Warm regards, Harv