Christianity and Islam originated from Judaism, the first religion known to teach the belief in one God. Islam took such a different path because of their Muhammad prophet guy, and Christianity did because of Christ, obviously. As for Jews, well, they're still waiting for their messiah. Therefore, they all have the same God, just totally different perspectives because of certain figures (or lack there of).
EDIT: Sorry for contributing to more religious talk in this thread. Last time, I promise-ish.
