Dr. Alan Rhoda presents it as the following trilemma:
Either (a) the atheist affirms that there is objective evil or (b) he affirms that there is none or (c) he remains agnostic on the matter.
If (a) then the atheist is committed to an objective standard of goodness, but whence does this standard of goodness come from?
If (b), then the atheist flies in the face of moral commonsense and gives up any objective basis for moral complaint.
If (c), then the atheist has the burden of explaining how it is possible that there be objective evil and also flies in the face of moral commonsense, which takes it as obvious that some things (e.g., torturing a baby for fun) are wrong.
That's a fairly clear example of a broader strategy I developed during innumerable college debates and bull sessions: try to get the atheist to paint himself into a corner he doesn't want to stay in. In that and later, more disciplined experience, I have found that most atheists eschew (b) as a matter of common sense. The smarter ones also eschew (c) because they don't want to be seen as failing to see the obvious. Which leaves us with the corner most of them, if they're sincere, will choose to paint themselves into: (a).
Here's what Rhoda says about that:
If, like nearly all modern Western atheists, he believes that the physical universe is all there is, then it is really hard to see where objective moral laws could come from. What arrangement of matter, energy, and space-time could give rise to a moral obligation? Of course, he can try to give an evolutionary explanation of why we have the moral intuitions we do, but at most that explains why we think that some things are moral and others aren't. It does nothing to explain how something could be moral or immoral.
While all that is perfectly true, the atheist has still some plausible options here. I know several, or think I do. But I invite suggestions and will post again on this topic once I receive some.
HT to Bill Vallicella.