This thread is, in a sense, a response to the frequent charge that atheism entails nihilism, relativism, egoism, or something like that. At the same time this thread is not about that particular entailment. Let’s just put that question on hold and ask ourselves: what does theism have to offer with regards to justifying morality?
So, let’s imagine that God has commanded moral system X. The question: is X moral because God commanded it, or did God command X because it is moral? Did God have some moral justification for commanding X, or is God’s command the moral justification for X?
Now, the second view has God as a sort of master philosopher, examining the universe and in ver infinite wisdom handing out the best possible sort of morality. On this view, theism might be useful, since it would be good to know what the right moral system is, but an atheistic universe would have just as much to offer with regards to justification. Whatever reasons God looked to in commanding X would exist without God - that’s the essential nature of this view. So here, theism has nothing special to offer with regards to moral justification.
The first view is a little more interesting. Ex hypothesi God had no moral justification for whatever moral system God commanded. But what other sort of reason could God have for giving these commands? Note that any sort of reason given cannot justify this particular command; it just explains it (for example, if I make a command, a non-justificatory explanation might be a psychoanalytic one). But I don’t think any such explanation will be forthcoming - feel free to correct me and provide some sort of non-justificatory explanation if you think you’ve got one.
That seems to mean that God did not actually have any reason for commanding the moral system God commanded. God could just have easily commanded anything - God could command us to murder our neighbors, rape our sons, worship Satan, anything. And if that’s the case, can we really say that the system God commanded is a moral system? Doesn’t it seem awfully arbitrary, in a way that an “objective” morality can’t be? So again, theism doesn’t seem to offer us any special way to ground a moral system.
A possible response might run something like this: “It is in the essential nature of God to command moral system X.” But I think that puts us right back into the previous view. Now God has reasons for this particular moral code, and if these reasons can justify God’s commanding it, then it could equally justify X in the absence of God.
I want to stress that though this thread is a response to questions about atheism and morality, we should not spend our time addressing whether atheism entails nihilism or relativism or whatever. That question is besides the point for the moment - we can resolve the issues I’ve brought up without discussing atheism and morality. That’s why the thread is called “Theism and Morality,” after all.