One caller, on a Catholic Answers Live program found here, recently raised the question that if God is good and moral, why does He allow evil and immorality to take place if He has the power to stop it. Much can be said concerning the subject of Theodicy, but what seems shortsighted on part of Atheists who ask this kind of question is: how does one determine what is moral and immoral apart from a perfectly moral standard such as God? If God does not exist, then how is it possible to speak of morality? In order to speak about a circular shape, the concept of a perfect circle must exist somewhere in order to have a perfect standard of circularity (in this case, the concept of a perfect circle eternally exists in the mind of God and has been implanted into our minds by God). Likewise, there must be a perfectly moral Being in order to determine what is moral and immoral. For this reason, when Atheists assume there is such a concept as morality and then use moral arguments to refute the existence of God, they are pulling the rug from under their feet since it leaves them with no ability to speak about morality, having excluded a perfectly moral standard by which they may judge what is moral and immoral.
At this point, the Atheist usually retorts with the assertion that societies determine morality. This is a very poor answer because it is clear that societies have chosen immoral laws throughout history, such as Nazi Germany’s laws concerning Jews or the Jim Crow Laws in the United States. I am not aware of many Atheists that would assert that these were morally good laws even though they were determined by a society. Thus, their assertion falls by the wayside.
Next time an Atheist presents this argument to you, turn it around on them and challenge them to provide you with a consistent reason they are able to speak about morality while denying the existence of a perfectly moral Being.