What is success for a religion?
I'd say it seems pretty fundamental that Religion is a kind of meme. One measure of success for memes is their ability to spread (virulence). If that is your only measure, you're likely to have some ethically terrible things going on. It seems (to me) like an obvious constraint that the meme must spread without causing any obvious harm in its wake (except to related memes that may be in competition).
What useful purposes does religion serve?
I consider [create an in-group] to be a pretty central (usually unstated) optimization point (not exactly a purpose) in most religions. This has some well-studied psychological effects on said group that can benefit all its members socially, psychologically, and (further down the causal line) medically. Although it tends to lead to some unfortunate side-effects for the out-group.
How would you design a "rational religion", if such an entity is possible?
Problem is that I've seen more than one source define "religion" as something like "systematic belief in the supernatural". I'm not convinced that such belief can be "rational" (optimal for making more effective decisions). Perhaps as a stepping stone -- use a religious-type belief system as an infection vector, then strip the supernatural elements away bit by bit; but that seems awfully deceitful to me.
This seems like a promising direction. So let's say that by religion I mean a useful meme. The meme consists of a doctrine. That is, a collection of statements regarding human belief or action. A person infected with the meme lives in accordance with the doctrine. The doctrine is designed to cause a useful effect, apart from its own flourishing. The effect is caused by changes in behavior of the people infected by the meme. The effect need not be explicitly stated within the doctrine. This a broad definition of religion, as it doesn't require that ...
This topic is vague and open-ended. I'm leaving it that way deliberately. Perhaps some interesting, better defined topics will grow out of it. Or perhaps it's too far afield from the concept of less wrong cognition to be of interest here. So I view this topic as exploratory rather than as an attempt to solve a specific problem.
What useful purposes does religion serve? Are any of these purposes non-supernaturalistic in nature? What is success for a religion and what elements of a religion tend to cause it to become successful? How would you design a "rational religion", if such an entity is possible? How and why would a religion with that design become successful and serve a useful purpose? What are the relationships between aspects of a religion, and outcomes involving that religion? For example, Catholicism discourages birth control. Lack of birth control encourages higher birthrates among Catholics. This encourages there to be a larger number of Catholics in the next generation than would otherwise be the case, Surely there are other relationships like this? How do aspects of religion cause them to evolve differently over time?