Perhaps the Best Argument from Religious Diversity

 ...is Jason and Jon Marsh's "The Explanatory Challenge of Religious Diversity."

Abstract: The challenge from religious diversity is widely thought to be one of the most important challenges facing religious belief. Despite this consensus, however, many epistemologists think that standard versions of the challenge fail because they threaten to implicate many seemingly reasonable yet highly controversial non-religious beliefs. In light of this we develop an alternative, less discussed, diversity challenge that does not generalize. This challenge concerns why so much religious diversity exists in the first place given common religious, and in particular theistic, views. Although there are some interesting scientific explanations of such diversity, satisfying theistic explanations of its existence are still required.

Required reading.