3. Myth provides the best explanation for objective moral values. If God does not exist, then objective moral values do not exist. Many theists and atheists alike concur on this point. For example, the late J.L. Mackie of Oxford University, one of the most influential atheists of our time, admitted, "If [...] there are [...] objective values, they make the existence of a god more probable than it would have been without them. Thus we have [...] a defensible argument from morality to the existence of a god."
But Mackie denied that objective moral values exist. He wrote, "It is easy to explain this moral sense as a natural product of biological and social evolution." Indeed, much of our moral drive is innate. Work by psychologists including Yale professor Paul Bloom has shown that even infants will show a preference for "moral" behaviour, and that we even apply our moral sense to inanimate objects.
Biologist Jerry Coyne sums up the current state of research in ethology: "Despite the notion that beasts behave bestially, scientists studying our primate relatives, such as chimpanzees, see evolutionary rudiments of morality: behaviors that look for all the world like altruism, sympathy, moral disapproval, sharing — even notions of fairness. This is exactly what we'd expect if human morality, like many other behaviors, is built partly on the genes of our ancestors." Primatologist Frans de Waal points out: "[T]he latest experiments in primatology reveal that our close relatives will do each other favors even if there’s nothing in it for themselves." And adds: "Mammals may derive pleasure from helping others in the same way that humans feel good doing good."
To think that there are any such intrinsic moral values would be absurd. What would such values be? Would anything humans do matter outside the existence of humans? More importantly, how does how we treat each other have anything outside of that interaction? The notion that there's some value of right or wrong intrinsic to our existence is a very queer one.
Yet this doesn't have the fatal consequences that the likes of William Lane Craig suggest it has. As empirical support, one only needs to look at attitudes concerning homosexuality or women's equality to see how well societies can change without any form of objective moral standard. It simply doesn't matter whether or not homosexuality is objectively right or wrong; what matters is that attitudes towards homosexuals have shifted in a positive direction without any sort of worrying about such silly notions as moral objectivity. Ethics in scientific research has improved remarkably in the last 100 years, yet no concern is given to whether or not it's objectively wrong to be unscrupulous in research.
Not having objective moral values doesn't mean that any action is of equal value, either. One would be perfectly justified in saying rape is wrong without having any notion of objective moral value. The notion of right or wrong is something we impose onto the situation. Most people, I would wager, would be content to say rape is wrong because of the harm it does, or the violation it imposes. And, really, what more ought to be needed? If that isn't enough, then what is? To come back to J.L. Mackie: "Morality does not need a god as a supreme source of commands or as a wielder of decisive sanctions. [...] There is no good reason for introducing a god even as an essential part of the content of moral thinking."
One more problem remains. Even if objective morality exists and is grounded in God, we have good reasons to doubt any claim to objectivity. On a sociological level, moral prescriptions are culturally-contingent. On a psychological level, different people espousing different moralities under the same justification. Believers who think that God is a source of morality tend to think that God's morality matches their own. From Epley et al. on the results of neuroimaging patients asked this question: "A final neuroimaging study demonstrated a clear convergence in neural activity when reasoning about one’s own beliefs and God’s beliefs, but clear divergences when reasoning about another person’s beliefs. In particular, reasoning about God’s beliefs activated areas associated with self-referential thinking more so than did reasoning about another person’s beliefs."
The same problem is encountered as with invoking God for causation or design - even if the beliefs are true they are indistinguishable from a projection of our minds. We have every reason to be sceptical of anyone claiming that they have God's objective moral truth; because it's indistinguishable from the illusion of it.