Despite all this scholars have studied religion, its evolution including the believers. The scholarly opinions are all over map because it is difficult to study religion as an objective phenomenon because of the subjective faith of the scholars.
Here is a talk by Dr. Jared Diamonds. His bio
Jared Diamond, professor of geography at UCLA, received the Pulitzer Prize for non-fiction in 1998 for Guns, Germs and Steel: The Fates of Human Societies. In 1999, he received the National Medal of Science. His most recent book is Collapse: How Societies Choose to Fail or Succeed (2004).He studies religion from evolutionary perspective.He believes that religions evolved along the changes in the human societies.
Professor Diamond argues that religion has encompassed at least four independent components that have arisen or disappeared at different stages of development of human societies over the last 10,000 years.
The talk is long but raises some very interesting points. Especially the religion providing justification for killing other humnas.