I will begin by assuming god does not exist and that we are merely examining the concept of god. As such, the concept must have been selected for in the evolutionary process, and if so, it must possess some survival value. Now people will be fairly justified to point out that its survival value may have initially been one of promoting group cooperation but that we now have other institutions that serve that purpose. So why does religion persist? The selection of the evolutionary process is ongoing, so it would not be consistent to assume it persists for any other reason than ongoing survival value. But what value beyond that provided by other institutions of group cooperation? There is one fairly glaring lack in all other such institutions. None offer a means to exercise (work to strengthen) the faculty of conscience. Only the concept of god provides a postulated observer whereby individuals can further develop an objective view of themselves. A postulated view that does not have the shortcoming inherent in all other human institutions that lead people to believe that something is only wrong if you get caught. Now we could imagine that 100% surveillance could serve the same purpose (someone watching you when you think no one else is looking), but we would also consider that an abhorrent breach of personal privacy. We could also be tempted to at least tell children fairy tales about such surveillance, but children tend to see through lies earlier that expected. Just like a doctor's delivery of a placebo can alter its efficacy, it would seem that belief in the concept of god (or an equivalent ever-watching observer) might effect its ability to strengthen conscience. Now I am open to any alternate suggestions of means to cultivating conscience. There may well be others, and I would be very interested if anyone can describe some. Obviously, our empathy for others can curtail behavior when in the presence of those others (knowing they will be aware of our misdeeds), but in perceived isolation only conscience suffices. So even if god does not exist but the concept of god may have a survival value selected for through evolution in the form of the faculty for conscience, this is very relevant, even to atheists. Atheists often complain about feeling isolated, ostracized, etc.. Others are evolutionarily justified in maintaining a mistrust of those who do not have any readily apparent means of developing their own conscience. So while developed social behaviors may well curtail behavior in public, others may have no reason to trust atheists to remain upstanding when unobserved.