The theist charge that atheists can’t have morality without a sky god is pretty laughable. But the error in the argument goes so deep that the argument is actually much more devastating when applied to theistic morality: how can a person following theistic morality be good?
Suppose your sky god decreed it morally good to steal, rape, and kill; and morally wrong to live peacefully. Would the atheists then be living immorally for abstaining from these activities? Clearly not. The theist argument presupposes that theistic morality corresponds to our intuitive morality. If theistic morality just codifies our innate moral sense, then it’s at best supplementing it; it’s not the source of morality.
The more interesting case occurs if theistic morality contradicts our intuitive morality. In this case, we say that god’s moral code is wrong, not that our innate one is wrong. So god is fundamentally constrained to codifying the morality inherent in human nature. If god deviates from that, we deem his morality to be wrong.
The problem with the theist argument is that morality is a collection of evolved instincts, not a set of rules passed down from on high (by gods or rulers). (I suggest reading Matt Ridley’s The Origins of Virtue for the argument from evolutionary psychology.)
So: if you follow intuitive morality, then you will act morally. If you follow theistic morality, then you may or may not act morally (depending on how closely the theistic morality corresponds to intuitive morality—usually very closely, otherwise it wouldn’t survive long.) The real question is this: if one is simply following god’s rules, how can we be sure that they will behave morally? As soon as god gives them the green light to steal, rape, and murder, they can override their innate moral qualms with religious justification.