Read another response by Stephen Maitzen
Is an act less moral if it is done by a Christian to an atheist? It seems to be this way because Christians only act morally because they're told to by god. Atheists have no need to be good but seem to act that way because they logically realise that it is the right thing to do. Not from fear of god/hell.
On this question, I doubt I can do better than to recommend to you an excellent article written by Professor Donald Hubin, available at this link: