The human idea of God has a history, since it has always meant something slightly different to each group of people who have used it at various points of time. The idea of God formed in one generation by one set of humn beings could be meaningless in another. Indeed, the statement "I believe in God" has no objective meaning, as such, but like any other statement only means something in context, when proclaimed by a particular community. Consequently, there is no one unchanging idea contained in the word "God"; instead, the word contains a whole spectrum of meanings, some of which are contradictory or even mutally exclusive. Had the notion of God not had this flexibility, it would not have survived to become one of the great human ideas. When one conception of God has ceased to have meaning or relevance, it has been quietly discarded and replaced by a new theology. A fundamentalist would deny this, since fundamentalism is antihistorical: it believes that Abraham, Moses and the later prophets all experienced their God in exactly the same way as people do today. Yet if we look at our [three] religions, it becomes clear that there is no objective view of "God": each generation has to create the image of God that works for it.
-- Karen Armstrong, A History of God
I highly recommend Armstrong's books A History of God and The Battle for God.
She's a scholar and fascinating woman -- a former nun, now self-described "freelance monotheist."
You can also listen to an interview with her on NPR's Speaking of Faith.