David Hume famously described the Is/Ought problem: there is no logical means of deriving moral values from statements of fact. Well before there was any evolutionary theory of morality, he recognized that our will is a "slave of the passions". Our motivations do not derive from reason alone. Through instinct, we make judgments about what is right and wrong. Through our sense of pride, humility, love and hate, we are motivated, and we experience social approval or disapproval as a result of our actions.
Theists too, generally agree with Hume's is/ought distinction - that moral imperatives cannot be derived logically. They insist that they come from God. They are divine commands that express objective facts known to us by revelation, rather than subjective judgments. The major difference between theists and naturalists in the logic of the is/ought distinction is really just a matter of defining what is meant by a moral statement. The Humean naturalist says that statements of empirical fact cannot be used to logically derive a conclusion regarding human values. The theists says that you would need a logical linkage between 'is' and 'ought' before you can draw any conclusion regarding moral facts. But moral facts don't come to us by a process of reasoning - they come from God.
Theists are happy to agree with Hume on the logic of moral reasoning because they think it provides a cudgel they can use against the perceived enemy they call scientism. They are indignant about the notion that science may try to intrude upon God's territory by deciding what is and isn't a moral fact. An example of this anti-science sentiment can be seen in this article by John Allen Gay:
We can also see an error in the systematic qualification of all moral judgments with the observation that these are the subjective feelings of a particular judge, such that morality is an entirely subjective phenomenon that lacks the broad, crosscultural and intersubjective force we’d typically think it has. The scientifically-motivated cultural relativist conflates scientific epistemology (that is, the ways through which we can arrive at knowledge) with truth, so that the only moral facts are objectively verifiable (i.e. scientific) facts. However, we need not conflate epistemology with truth. Surely there can be facts that we cannot verify, period, let alone verify them with scientific methods.In this case, the theist conflates scientific knowledge about the sources of human morality with a scientific approach to defining our moral values. There aren't many scientists who think that science should be used to define what our moral values should be. Even Sam Harris, with his scientific approach to answering questions of morality, must start from the assumption of a fundamental moral value as an objective fact: that the morally good is something that increases the overall well-being of conscious creatures. But aside from the obvious objection that it may not enjoy universal agreement, this assumption violates the spirit of empirical science, and so will probably never be accepted in the broader scientific community as scientific fact. There doesn't seem to be much of a threat of science dictating what our moral values should be, so I think the theist has little to fear from this. But what he really fears is a scientific understanding of morality that contradicts his religious belief that moral truths are objective facts that come from God.
Religionists make the mistake of thinking that their moral values are objective facts despite the patently obvious reality that they are value judgments. There is no universal agreement about them. Gay says:
There are two sorts of facts about the world, which I’ll call standard facts and moral facts. Standard facts are characterized by the use of the verb “is,” as in “The sky is blue” or “Two plus two is four.” Moral facts are characterized by the use of some form of “ought,” which indicates the feeling of duty or obligatoriness that characterizes moral sentiment.No. There are facts and there are opinions. For any particular moral value proposition, its acceptance among the general populace could be anywhere in a range of "unique to an individual" to "accepted by the vast majority". Most moral propositions fall somewhere in the middle of that range. There is certainly no universal God-given standard to which everyone can appeal. And this fact alone is testimony to the subjective nature of morality. If moral facts did indeed come from God, it would stand to reason that he would tell us all the same thing, but clearly that's not the case. Reasonable people have significantly different opinions about what is morally right. Despite this, religionists continue to insist that their moral opinions are objective facts.
If you ask them to tell you definitively what these objective facts are, the typical response is "It is wrong to rape babies". What they're doing is to choose an extreme case (at the high end of the range), where virtually everybody agrees, and then pretending this settles the issue, as if all moral questions enjoyed a similar level of agreement. It may be the case that there isn't a single person in the world that thinks it is OK to rape babies. But that doesn't change the fact that this is still a value judgment. It just happens to be one that most people agree about. The reality is that for the majority of moral values or propositions, there is considerably more disagreement than that. In light of this, it's not reasonable to think that these propositions are objective facts.
A better way to look at the issue is to recognize in accordance with Hume that it is quite true that you can't derive 'ought' from 'is', for the simple reason that facts cannot be used to prescribe what our values should be. But rather, we understand that if one wishes to achieve some goal (based on a value judgment about what is good or desirable), then one ought to take a course of action that promotes the desired outcome. This provides a logical linkage - not between 'is' and 'ought', but between values and actions. If we have a particular value A, and we wish to achieve a realization of that value, that constitutes a goal. The pursuit of that goal entails a course of action B. We can then say that if we wish to achieve A, then we ought to take action B.
Of course, this formulation makes no attempt to define what A is, nor should it. But it does leave open the possibility of using science to help us provide a means to achieve our goals. Still, I harbor no illusion that religious believers will suddenly adopt a more rational approach moral reasoning.