Materialism could have taken over thousands of years before, so I don't think this reasoning makes sense.

What seems more likely (to me) is that due to advancements in transport and information technology people increasingly learned about different world religions, and did the math.

If there is only one God, how can other people worship a different one who is also the only one?

Also, religion started to fade during The Enlightenment, and things took a little while to completely crumble.