I've written papers on this topic. You are correct on both counts @Karnage .

Geographic and linear birth determine religiosity. Christianity has survived and thrived due to the medieval architecture that survived and the remnant priesthood that had to be maintained. Earlier religions did not have these reasons to continue.

If you were born in India you would grow up believing in Ganesh or another of the many gods. If you were born in Scandinavia you grew up with a belief in Odin, ect.