I have to disagree with you. IN the first place, the god of Judism, Christianity and Islam is the same. But that is not surprising since they all spring from Judism.
But even in the religions that profess belief in multiple gods, Hinduism for example, there is a common thread. Hinduism, which has many Gods, really has 3 major gods (ah! The concept of the trinity again!) and one of them is God of all gods! Greek and Roman mythology had the same set up. So are they really that different from the Judeo-Christian god?
I think not. For you can view the lesser gods in most religions as angels in the Judeo-christian religion. When you break down the major religions, you begin to see a lot of commonality between them. Is that because of the limited nature of man and his imagination? I doubt many would subscribe to that theory. SO there appears to be something more behind the commonality.
For some, that is the belief that there really is a god. For others, it is just the natural progression of human logic (everything coming to a central point - or in the case of religion,to 1 over arching surpreme being).
What I find fascinating about this point (and I do agree with it) is what happened to make the major religions of today so long lasting? We call the religions of the Greeks and Romans 'Mythology', yet they are really ancient religions that did not survive their civilizations. Yet Hinduism, Buddism, the trio of Judeo religions (for a convenient grouping) have all survived thousands of years when the normal life span of a religion is but a couple hundred.
And I ask why? I know that each suceeding generation molds their religion to fit the times. Indeed, if you were to point out the one major purpose of the Catholic Church, it would be to stop that from happening. And even they are not totally sucessful (Note Vatican II in the early 60s). Man always tries to mold the religion to fit their current world problems. The debate about gays was not an issue 200 years ago, nor was the issue of Abortion. Today they occupy an inordinant amount of time in the politics of most religions.
So while the major religions have not changed in the last 1500 years, all of them have changed to fit the times. (Confusing, sorry. I mean the major religions today are the same ones from 1500 years ago.) Yet with the changing, they still, at the core, are the same.
That is the $64 question. Why do these religions survive and thrive, yet others wither and die? Perhaps because the ones that contain some of the absolute truth will transcend their creators?
I dont know. But is is an interesting question.