It's a common argument against atheism that atheists cannot have a morality or moral code on a philosophical level, as they do not have a series of divine instructions the blindly obey. But the truth is that atheists get their moral code from the same sources that the Western religious do, and that is not the Bible.
Serious thinkers look at different historical trends and notice some patterns. Societies that prohibit murder, for example, are more stable than those that do not, or only prohibit some murders. Societies that respect property rights have more wealth than those that don't, which tend to starve.
Another way to look at the issue is that there is a societal evolutionary process. Societies with useful laws and morality thrive and pass on their laws and moral codes to other societies. Societies with poor moral codes and laws don't succeed or pass their values on. Rome was especially powerful and successful in passing in values and codes along to other societies. The Golden Horde and the Mongols were not successful, despite their power.
Likewise, on a personal level, certain moral codes produce superior results. A moral imperative to work hard and be honest, for instance, will generally result in success, where-as sloth and dishonesty results in disaster in the long-term. A moral code that openly supports deception against other groups breeds ill-will and mistrust against the other groups, equally individuals who act dishonestly held in a similar fashion.
The development of an areligious morality is a fairly straight-forward review of what creates the most comfort and safety in the world. And "Christian" morality is the same - the things that produced good things are what Christian kept, and and those things that were bad Christians ignored or read out of the Bible. Most Christians are not selling off basic necessities to purchase weapons, for example. They, like most everyone else, take what has worked and left what hasn't.
As a good counter-example, we can look at Islam. The historical trend of Islam has been one of aggression and conquest, eventually resulting in terrorism, murder and general thugishness seen today. It is only through divine command that people would engage in directed, disciplined, pointless and self-destructive violence. No rational morality could justify random suicidal violence, but this supernatural authority commands it, and people obey.
And that's the ultimate irony about the claim that atheists can't have a moral system; the historical truth is that most of the Western moral code was influenced by Roman, Greek and British sources independent of Christian doctrine. What's more, religious codes of morality are not always good, and sometimes even truly evil.
And as a final note, to say "God is good" or to speak about a "Good God" is already to place a moral value above and outside God.