hungarianelephant:

How do you show that actually being prone to religiosity, as opposed to being good at feigning it, is an advantage?

Groups of animals almost logically demand arms races in deception and perception:
1) Animals living in groups will favor genes for reciprocal altruism.
2) Genes that mimic altruistic behavior in a manner that is less costly than actual altruism (deception) will do better than genes that dictate actual altruism.
3) Given some level of deception, genes that are better at spotting the freeloaders (perception), will do better than genes that aren't.

This gets us to a group of people that both have genes for deceiving and perceiving deception. They have also evolved means to communicate their intents.

I posit that it is more difficult to perceive that a person is deceiving you, especially if the deception is regarding their intent, if that person herself believes that they are being honest. And so, genes that promote self-deception along with deception become more common.

In this way, evolution will select for a person who strikes the best balance between lying to others and appearing honest (sort of like politics). If those who sincerely believe their own bullshit appear more honest than those who don't (and I think that's true), then the self-deluded swindler should be an archetypal behavior pattern in all human groups. Preacher, anyone?