This is true. There is indeed a secular man-made Oprah Winfrey "religion" that says "there couldn't possibly be only one way"
You are correct to deduce that every religion can't have it's own god. There can be only one true God.
Assuming that is true, then it wouldn't be surprising if he decided that there is only one way to meet his acceptance criteria. (Try getting into a university without meeting their application requirements.)
So, look at every belief system and see where it comes from. If it is a single self-proclaimed prophet, watch out.
It stands to reason that if God had rules he would communicate them to mankind as early in our history as possible and preserve a consistent record of those communications down through history.
See if you can find evidence of that. :o)