Ever since the 2000 publication of Ted Lockhart's Moral Uncertainty and its Consequences, philosophers have become increasingly concerned about the problem of reasoning under moral uncertainty. Even after a long period of deep study and reflection, many people are uncertain about which moral theory is correct, and even about when one moral principle is weightier than another. Many people are also uncertain, even after a long period of deep study and reflection, about the moral status of at least non-human animals and insects. However, there are many ordinary moral contexts where the moral rightness or wrongness, or goodness or badness, of an action depends on which moral theory is correct, which moral principle is weightier than another, or whether non-human animals and/or insects have moral standing. Furthermore, often the choice between acting on one moral theory (or principle, or view about the moral standing of non-human animals and/or insects) is such that if we make the wrong choice, we've done something horrendously bad or wrong. Here's a simple intuitive example from Carr (2020):
Go Vegan? Sally is uncertain about whether non-human animals have moral standing. She is certain that, if animals don’t have moral standing, it’s a little better for her to eat meat, eggs, and dairy occasionally for gustatory and social reasons. She’s also certain that, if animals do have moral standing, it’s badly morally wrong for her to eat non-vegan foods.
The phenomena of moral uncertainty of this sort and others is highly surprising on theism, since it is widely held that on that hypothesis, one major purpose for humans in this world is that of exercising our moral freedom to make serious moral choices and to develop our character. In any case, at the very least, it's surprising on theism to find the world to be one of moral Russian roulette. By contrast, the phenomena of reasoning under moral uncertainty is not surprising on the hypothesis of naturalism, since on that hypothesis, there is no personal god who has made the world to be an arena for exercising moral freedom and character development. The phenomena of reasoning under moral uncertainty therefore constitutes at least some evidence for naturalism vis-a-vis theism.