Since Ryan claims the objective of his book is to respond to bad arguments, I can’t find too much fault with his response to the form of the Moral Argument he quotes in the second chapter. However, not having seen or heard this form, I can’t help but wonder if there isn’t a Straw Man under attack here.
I am not aware of any theists who justify belief in God by the existence of a concept of morality. Rather the fact that we have moral intuitions leads us to ask what grounds them.
Ryan offers an evolutionary explanation for moral codes, but not one for moral intuitions. If all we had were rules we had to learn, he might have a point, but there are good reasons to think we have moral intuitions before we learn moral codes. As parents we often appeal to a young child’s empathy to correct their behavior. However, that empathy does not account for the “oughtness” of acting according to it. Just because I can imagine how it would feel to be the victim of my actions does not explain why I should not do those actions. In fact in some cases, I ought to do them anyway.
Ryan points to cultural differences as evidence of relativism, but these differences are far more about the facts that inform the application of moral principles than the principles themselves. He notes an example of an objective moral principle, the wrongness of murder, but jumps quickly back to denying a logical connection to theism.
Comparing moral codes of various religions adds nothing to the discussion since revealed codes are not central to the argument.
Finally, since moral arguments are defeaters for atheism, and not arguments for a particular deity, noting that they do not prove one as more plausible than another is a red herring.