It occurs to me that the primary purpose of religion for the believer is to be a moral guidance system. Thus, the greatest threat to religious belief, and the weakest argument against it, is the advocation of a relative moral standard. Saying that what is good or bad depends on context, as true as that might be, is like saying, "I recognize that you seek moral guidance in religion because you feel that humans are naturally incapable of being moral, but I have found that there are no moral absolutes. Your search has been in vain. Morality is context-dependent or relative. Sorry, but you should give up and reject belief in your deity."

In this light, it is easy to see that this tactic will almost always backfire on atheists. Therefore, the only argument for a purely natural morality is a lived one. If you demonstrate that you can live a moral life without belief in a supernatural entity or force, then you will prove that no supernatural entity or force is needed to guide us in making moral decisions.