…if there isn’t any God to issue commandments to us, then why think that we have any moral duties? On the atheistic view, human being don’t seem to have any moral obligations to one another. For example, in the animal kingdom, if lion kills a zebra, it kills the zebra but it doesn’t murder the zebra. If a great white shark copulates forcibly with a female, it forcibly copulates with the female, but it doesn’t rape the female, for there is no moral dimension to these actions. None of these things is prohibited or commanded; they are neither forbidden nor obligatory. So if God doesn’t exist, why think that we have any moral obligations? Who or what imposes such prohibitions or obligations upon us? Where do they come from? It is hard to see why moral duties would be anything more than the illusory by-products of social and parental conditioning.
. . . If there isn’t any moral law giver then there isn’t any moral law that imposes itself upon us.
–William Lane Craig