I think that is a question we will have trouble answering in the centuries to come. Many religions wants to claim it's origins with the birth of man. In truth, the best I can say is:
There are carvings and paintings worshiping the female form as the giver of life (or so archaeologists think) from many thousands of years before recorded history.
As far back as I have been able to find, Pagan religions existed long before any form of Christianity (though, don't tell a Christian that or you may get burned at the stake. That was a joke people.).
Look up Mesopotamian religion and you will find the most widely accepted information on the oldest religions.
And finally; there are cultures that existed 12,000 years ago (look it up before you call me a liar) and we have no way of knowing what they believed but speculation has already run rampant.
Even if we were to all decide that religion was no longer enough by itself, I am not sure that our personal biases could truthfully answer which religion really came first.