4 Answers

Man spoke first and then much later started to write things down, so there really is no way of knowing the answer. There are several religions that no longer exist. An example = Mithraism ( practiced during the late Roman Empire era). We are just starting, what is called, the 5th era of man. It should be intersting to see what religions servive this era.

Idolatry.

It is said that even cavemen had some sort of faith for the beyond. They have found where cavemen would throw their dead down into a deep cave/hole and they have found things of importances next to these skeletons like spearheads or axes etc, etc. It is possible but not known that the cavemen may have looked up to the sky and made an offer and threw these things down to their loved ones.

the first organized religion, I'd say Judaism..