You said,
"So a Christian would say that 'saving your soul' is even more important than surviving, in some respects - that, for example, if what you did in service of survival, endanged the destiny of the soul, then you ought not to do it. But that is not a matter for biology." I agree, but you are overlooking something and are looking too much into individual matters. You aren't understanding what I'm trying to explain in my original post, probably because I edited it after you replied to it.
When birds look for a mate, they don't consciously think "who has the best genes or who will make strong, healthy, and attractive offspring." They just look for a pretty bird and fuck it. Humans do the same thing, sometimes (not with birds). Even though fucking is not centered to surviving in this perspective, it plays a vital role in the survivablity of a species because it's the natural way of mixing genes, getting mutations on offspring, and increasing the population. Animals and humans do and think things for whatever the reasons are, but biology and psychology analyzes them in a different level of thinking.
Okay, now let's apply this to religions. The central point of religions obviously isn't about surviving with a bunch of sweat and blood on one's body. Religions helps people socialize, which I kept saying. People group up in churches, help each other, interact with each other, offer advice to each other, talk about the bible or whatever, offer services to each other, offer ideas about the religious texts to each other, gain knowledge about the people there such as their names, and etc. So there's obviously some objective gains in joining a church besides going to heaven or giving a damn about God. These objective reasons can influence surviving in our societies greatly. For example, having a friendly teacher that goes to my church helps out my son with his math homework (I don't have a son yet).
If someone criticizes the bible heavily, s/he will most likely be socially exiled from the church and thus won't receive any support from the people in that church. This is bad, not because s/he will go to hell or w/e, but because the person no longer has access for help from the people in that church.
Anyways, I was trying to illustrate how objectively important or impactful religions and churches can be for the social networks of societies in the social level. Now back to what you've said.
You said something like if someone did something in service of surviving, such as killing and eating a cute bunny, but is against the rules of the religion to do so and will be punished severely by going to hell (or soul is devoured, w/e), then the person will not eat the bunnies and this isn't a matter of biology. Actually it kind of is because the bunny population will most likely not be endangered due to humans. Also, if the bunnies were poisonous, people wouldn't die from eating them because they don't want to lose their souls. People may use bunnies as a religious figure for ceremonies and thus may interbreed a lot of them and use as pets. Humans are so powerful in this world that simple religious ideas can influence other species, the habitats, and even humans themselves.
A more realistic example is an oracle telling young men that their fates will be hell if they stay in their homelands and that their destinies lie beyond the waters. Thus, the men will be very inclined to travel to beyond the waters, despite how dangerous it is and how low their probability of finding land is, and search for another island or piece of land out in the large ocean somewhere.
If it wasn't for that crazy oracle, no one would have populated those empty (no human zone) islands beyond the waters. This is a huge survival advantage for the people that got to the island because then they have access to more resources, food, and land. The oracle obviously wasn't thinking, "What can I say to these morons so then they travel to those islands and give us a huge survival advantage." The oracle was probably high from smoking drugs, has a psychological disorder of some kind, was trying to scare them away because the community is low in resources, or a mixture of reasons.
Philosophical systems such as religions influence groups of people's minds so greatly that it can have such a huge impact in the world that it can end up influencing the biological parts of the world as well because humans already have a lot of power over other species on this planet.
Another example that is close to modern times is when Christians were saying only gays have HIV and that it was a creation from God to punish gays. Thus, straight people were having unsafe sex frequently and were spreading HIV like wildfire. Now we have a bunch of people with HIV now.