Another aspect of the human condition which is much easier to understand given theism rather than atheism is that we are burdened with a deep sense of moral obligation. As human beings we strive to ground morality in something more solid than our own subjective preferences, but if there is no God there is nothing else upon which to base it. In a purely material world morality is nothing more than whatever feels right to the individual.
This is not to say that the non-theist cannot live a life similar in quality to that of a theist. She can of course, but what she cannot say is that what she does is morally good or right. There simply is no moral good unless there is an objective, transcendent standard of goodness, and the existence of such a standard is precisely what non-theists deny. Consider these two quotes from some well-known atheists:
"In an important sense, ethics as we understand it is an illusion fobbed off on us by our genes to get us to cooperate. The way our biology forces its codes is by making us think that there is an objective higher code, to which we are all subject." E.O. Wilson
"Life has no design, no purpose, no evil and no good, nothing but blind indifference." Richard Dawkins
For the atheist moral judgments can be little more than expressions of personal preference, and no one's preference is any more authoritative than anyone else's. This leads ineluctably toward a might makes right egoism, either on the level of the individual or the level of the state. Whatever those who control power do is not morally right or wrong - even if they commit torture or genocide - it just is.
Moreover, unless there is a transcendent moral authority there is nothing whatsoever which obligates us to act in one way rather than another. What could possibly obligate me, in a moral sense, to act in the interest of the collective rather than in what I perceive to be my own interest? Given naturalism, there is nothing which obligates us to care for the poor, nothing which makes kindness better than cruelty, nothing, indeed, to tell us why the holocaust was wrong.
Given atheism, morality is either subjective, and thus arbitrary and personal, or it doesn't exist at all, and our sense, our conviction that it does is simply self-deception. If God exists, however, then, and only then, does our intuition that objective moral value and obligation also exist make sense.
Related to the matter of moral obligation is the question of guilt. We experience feelings of guilt, and have a sense that guilt is not just an illusion, but without an objective standard of morality before which we stand convicted there can be no real guilt. Human beings are no more guilty in a moral sense than is a cat which has caught and tortured a bird. The feeling of guilt is merely an evolutionary epiphenomenon which arose to suit us for life in the stone age and which, like our tonsils, we no longer need. Indeed, it's a vestige of our past that we should suppress since it bears no relation to any actual state of affairs.
On the other hand, if there is an omniscient, omnipotent, perfectly good Creator of the universe, then our sense that we are actually guilty has an explanation. We feel guilt because we have transgressed the moral law instituted by the Creator before whom we stand and to whom we must give an account.
It is this Creator who imposes upon us moral obligation. Take away God and there's no moral law, there's no moral obligation, there's no transgression, and there's no moral guilt. As Dostoyevsky put it, if God is dead then everything is permitted.
RLC