Personally I haven't a clue. However people join religions every day. Something makes one theistic religion more attractive than another, what is it? Once a religion of choice is made then I'm thinking there must be something about all the others that makes them less appealing.
Not every religion began at the same time, so what went wrong, why have some disappeared altogether? Is the first religion still with us and if not, then why would it ever disappear? You would think the first religion would be closer to getting it right, seeing how they would be early enough to observe the god(s) in action. Still, the time of origin doesn't seem important since some religions are more recent than others.
Hard to believe the first theists got it wrong. If you say they did then what's that tell you? If we are talking about a search for the truth then wouldn't it be fair to say that the farther in time you are from the origin of the first religion the less chance you have of finding it? Why would the first religion become so unattractive? Why would the god permit it? I find it odd that the first religion didn't stick. It may still be here but why has it been shunned by many?
It doesn't feel right. I smell human intervention. How does evidence for god change from one era to another? God isn't a new idea, so why isn't the most ancient theistic religion followed? Is it that no one including theists believe God left any evidence for the first theists? Interpretation? How did the first theists know there was a god? Were they wrong?