Moral duties must inevitably be grounded in the subjective desires and preferences of at least one moral agent, since there can be no values without minds. Theists generally prefer to ground moral values in a mind which has the virtue of being inhuman, immaterial, atemporal, eternal, etc. because such a mind presumably holds values and virtues which are immutable and incorruptible. Does this imply that such values are necessarily (or even probably) preferable to our own?
One may easily conceive of something like an hypothetical hyper-powerful evil demon, who creates whole worlds simply for the sake of giving its inhabitants desires and hopes and then inflicting suffering upon them. Would we consider this sadistic demon’s moral values preferable to our own merely because they are independent of humanity, space, and time? If not, why not? On theistic ethics, it is only the objectivity of moral values matters, without any regard to human desires and values.
Divorcing ethics from human needs and values, however, inevitably muddles up the whole enterprise. Some crimes cannot even be defined and evaluated without an evaluation of human desires and intentions. What looks to an outside observer like torture and rape may actually be sadomasochistic role playing, understanding the intentions and desires of the participants is absolutely necessary to evaluate the morality of such actions. Similarly, what looks like murder may turn out to be assisted suicide, once we discover the intentions and voluntary cooperation of the deceased. In such cases, the criminality of the acts may turn entirely on the subjective values and intentions of the alleged victim.
By way of contrast, such crimes may be wholly justified on theism, without any regard for the human victims involved. On the theistic view of ethics, if the gods command you to slaughter the natives and keep the virgins for yourselves, you must cast aside your own subjective distaste for murder and rape in order to put faith and fealty first and foremost. This strikes me as an incredibly immoral approach to morality, but hey, at least it is objective rather than rooting itself in such pathetic human impulses such as love, mercy and empathy.
It seems clear enough to me that the whole point in having moral intuitions and creating moral norms is to make life better for the humans who must get along together. On metaphysical naturalism, this is more or less the only plausible account of morality. Many devout believers claim that it is impossible for them to conceive of any reason to work for a good life if there is no transcendent being offering moral norms carved in stone and the promise of eternal rewards. This is, I believe, almost inevitably an illusion. There may be some for whom apostasy implies an immediate descent into sociopathic amorality, but I am quite happy to say that I’ve not met any of them yet, despite having known many scores of freethinkers.