When questions of moral imperative arise, I have a friend who likes to jokingly inform me that as an atheist, I am not bound by any moral code and can therefore do whatever I want. If a coworker is annoying me, I am free to stab him in the back or run him down with my car, my friend reasons. Likewise, if a neighbor owns a house or vehicle that I lust after, I am free to take it for myself. As for keeping the Sabbath holy, apparently I and other atheists are the only ones permitted to work on that day (which makes one wonder why there are so many Christians working on Sundays). According to my friend, atheists are free to act selfishly because they do not fear the retribution of God.
One thing that bothers me about this line of reasoning is the implication that the source of human morality is obedience to authority and fear of punishment. Yet the fact that we draw a distinction between legality and morality shows that authoritative pronouncement in and of itself does not make and act moral or immoral. If it did, then the fact that we have laws against acts such as murder and theft, and that those caught perpetrating these acts are punished, would be enough to provide me and other atheists with a binding moral code. But clearly it is not. Although laws often echo deeply held moral convictions, there are many cases in which they do not. The correlation between law and morality is far from absolute.
Even the claim that God's existence outside of time and space ensures that morality is absolute doesn't hold water. To non-Christians, a lot of what passes for moral behavior in the Bible strikes the rest of us as either immorality or moral relativism. "Thou shalt not murder," God commands. Yet the Bible is replete with murders convicted by him or on his behalf, including the murder of the firstborn sons of the people of Egypt. Intentionally killing infants because they are the offspring of a people whose leader you have a disagreement is both murder and punishment of guilt by association. The definition of murder thus appears to vary according to God's whims.
The same can be said of incest, which is not prohibited until Moses' time. Furthermore, what is defined as sin changes after the Fall of Adam and Eve, and again after the New Covenant. But perhaps the most offensive example of moral relativism is the fact that God does not hold those baptized in the Christian faith to the same moral code as those who are not. To a non-Christian, the fact that those who believe in and accept Jesus as their Savior can literally get away with murder while the rest of us would be condemned to Hell for even less seems nothing short of unjust. We would not hesitate to criticize a national penal system that freed criminals who were friends of and swore allegiance to the President but left the rest to rot in prison, so upon what basis would God's penal system be deemed any fairer?
Not only does biblical morality vary, it also endorses behaviors which we now perceive of as being grossly immoral. After centuries of rationalizing the act of treating other humans as property with no rights of their own, we now find slavery morally repugnant. The fact that the Bible not only permits but regulates slavery violates our own sense of moral judgment as well as the Golden Rule. On the other extreme, the Bible very often applies a punishment that far exceeds the crime, including capital punishment for working on the Sabbath or for a child who curses a parent.
Christians are good at rationalizing away these inconsistencies, but they wouldn't need to rationalize if biblical morality were more consistent in the first place. In fact, biblical morality is sufficiently ambiguous that even among biblical scholars there are multiple interpretations. With a moral code that is that difficult to understand and follow, what sense does it make to claim that Christian morality is absolute?
Whether my friend wishes to believe it or not, atheists are every bit as likely to act morally as Christians are. Although atheists may not have a book of scripture to consult regarding moral matters, most atheists are guided by humanist principles and strive to employ logic and reason rather than undue appeals to authority or emotion when making moral decisions. In short, those who uphold the Christian faith are no more bound to follow any particular moral code than the rest of us are. And were I given the choice to be surrounded either by those who are guided by the principles of humanism and logic or by those who claim to be acting in the service of God, I'd choose the former every single time.