I am redating this old post.
One interesting point about many ethical philosophies is that while they make no reference to a theistic God, they do seem to be grounded in metaphysics, and the kind of metaphysics at work is one that a modern naturalist would have trouble accepting. Plato, Aristotle, and the Stoics are prime examples. Plato's moral philosophy is based on the Form of the Good, which shares certain characteristics with the theistic God, and which can be known through a process of recollection where we recollect what we were aware of in a pre-existence. Aristotle is based on the idea of an inherent purpose for human life, and Stoic ethics is a response to Stoic metaphysics. No one seems to be suggesting that ethics will be all just the same regardless of metaphysics. Even if a personal God isn't required for ethics, doesn't it seem plausible that at the very least some sort of metaphysics is required that most naturalists today would have a hard time accepting. Is it reasonable to reject what Kant called a metaphysics of morals?