Can a theistic faith be considered an evolutionary advantage?

yes of course. It also helps to give cohesiveness to a group which can make them better in collaborating. Better collaboration means more likely to survive.

And don't forget that if belief in a sugar pill can cure a lot of ills (all medical experiments have to control for this, the placebo effect can be as high as 50-70%) belief in god should be able to cure as well as the sugar pill. So if believing in some kind of god can cure at least some ills, you are going to live longer which means you could have more children or look better after them so they are more likely to survive.

I don't think you really need god for ethics though. I don't think it makes too much difference to add gods retribution (especially if it turns out to be not immediate but after death) to the certain retribution by your fellow humans. How many people don't get drunk today to save themselves from a headache tomorrow?

Also there seems to be some evidence that spirituality (not a specific religion) is inheritable (evidence is from identical/fraternal twin studies)

Yes it could be. Not so much a physical thing but as far as sexual selection (who you choose to marry) it can be a big part. Many people will only marry those in their own religion (weather its right is not the question, the fact is it happens). Also evolutionary speaking the most important thing to do is pass on genes to offspring. So put together point one and 2, what do you get? Belief in a God can be an evolutionary advantage

I think that it may provide an evolutionary advantage in producing group solidarity. Groups that had a religion might survive better than groups that didn't, and individuals in those groups pass on the traits that produce religiosity. As far as promoting ethical behavior, you don't need a religion for that. You can be ethical without being religious. You need ethics, after all, to survive in a social setting.

Moral relativism does prevail. It prevails within religions and without it. And that morality seems to improve as time goes on. Slavery has gone from acceptable to reprehensible within the context of the history of Christianity. The NT provides rules for the proper upkeep of slaves; no Christian would find slavery acceptable today.

I'm not positive your first statement is correct. Most Gods punish people who kill other people within the tribe, but permits (or even requires) killing people from other groups. The OT God suggests killing for a variety of reasons, from adultery to impertinence, and commands that the Hebrews kill various outside groups of people. Saying 'thou shalt not kill' (or murder) at one point and encouraging killing elsewhere in the book sort of tends to undermine any religious authority - do as I say, not as I do; do as I say here, not as I say there.

In a sense, natural selection would most favor this kind of society, however, since they are trying to forbid killing of in-group members, unless they disrupt society, and encourage the killing of out-group members. A more purely ethical society, one that says, "Don't kill anyone, for any reason", doesn't get the differential advantage that a "Kill them, not us" group gets.

I believe theistic faith can be an evolutionary advantage but I do not think it was a result of natural selection.

Name a single secular society that says murder is an okay form of expression.

Being Jewish in Nazi Germany was an evolutionary disadvantage, because the Nazis exterminated the Jews...

Due to evolution? Definitely. An advantage? Certainly not.