I was responding to a comment on Power to Change's website just now and had this thought … not sure if this argument is valid, sound, cogent, etc, but I think it's at least interesting. I'm quite certain I must've read it or heard something like it before but I'm not sure where.
An attempt to argue that naturalistic systems of morality are innately inferior to theistic systems:
Any naturalistic morality system is ultimately unjust, and therefore immoral. Here's why: Human beings rightly crave justice, and any system of morality that is unjust would be by definition immoral. But if there is no afterlife (and therefore no final accountability for a person's actions), then life itself is ultimately unfair since good deeds will often go unrewarded and bad behavior will often go unpunished. Therefore, only a moral system that includes an afterlife (and by implication, God) where justice regarding a person's actions can be appropriately meted out can be just. Any moral system that does not is immoral and therefore deficient.