I've been mulling over this question some more...
Although most atheists probably have a morality very similar to Christian morality (e.g., do not murder, do not steal, etc.), they philosophically arrive at it by entirely different means and for different reasons.
I don't want to speak for all atheists, but this is how I approached morality when I was an atheist: first, I begin with a premise, such as "life is better than non-life" and "suffering is bad". I then tried to logically construct what was most supportive of these ideas--such as prohibiting murder or torture. Sometimes these premises would conflict (e.g., someone to whom life is just endless suffering), and so I would try to solve the moral delima in a way that the "highest" good was achieved--thus creating a heirarchy in my premises--or at least giving "veto power" to certain premises over and above others.
Yet one thing was clear to me, even then: my premises were arbitrary. Sure, I can understand why I came up with certain of those premises--if my ancestors didn't (for example) think life was good, then they would probably not be as apt to survive as those who did value life, and thus that line would die out before I came along. But just because evolution supports a certain idea ("survivors survive"--kind of a tautology, but useful nonetheless) does not make it "better" or "worse" than any other idea. My preference for life over death, for survival over non-survival doesn't really matter to the universe--it doesn't care in a materialist world view. I really didn't have the right to become morally indignint when others came to different conclusions, because there was no "metaethic" that said I was right and they were wrong. Stalin and Mao had their arbitrary values; I (and most of the rest of humanity) and different ones. But who was "right" and who was "wrong"?
As a theist, the who question is completely different. If I accept the premise of a Creator (note: I'm not talking about creationism--just a creator who may have worked through evolution, but nonetheless the evolution was directed to an intended goal), then this Creator would have designed the Universe (and me) in a particular way with particular abilities. Now, a generic Creator may not have any kind of "morality" as we conceive (or even what we might consider an "evil" morality), but nevertheless this Creator designed things a certain way. For example, he created the "natural laws" which everything must obey--gravity, E=MC2, etc.
Furthermore, as not just a theist but a Christian, I believe that that design included a moral law, which is different from other natural law in that we can choose to ignore it if we want to. Nevertheless, by ignoring it, we are deviating from our created purpose.
Indulge me in a fable for a moment: imagine if we were all eagles who had wings to fly, but also legs to walk (although the walking is not very good). And also imagine, for whatever reason, some eagles got the idea that the wings we have really made walking difficult and thought we would be better off without them. These eagles would train themselves to walk better, ignore and perhaps even remove their wings, and tell all the other eagles how silly they were for wanting to fly rather than walk.
I think the point I'm driving at should be obvious: what the Christian believes--and the atheist rejects--is that we have the wings of moral law, and we are designed to fly with it. To the atheist, this is silly, believing that the wings aren't even really--it's just an arbitrary decision. (Or at least that's my impression of how at least some atheists think.)
I guess to sum it up, an atheistic morality is one where the values and goals are arbirary, but a theistic (specifically Christian) morality is one where morality is part of the design of the Universe. (It's not merely the arbirary decision of some deity, either--even if some Christians argue that way. It's the nature of the Deity, manifest in the nature of creation).
Anyway, God bless!
For me now as
