if you sat in a debate ring with a serial killer, dear atheist, you wouldn't be able to prove to the audience that what he does is morally wrong
Of course not, you couldn't even 'prove' you don't live in the matrix. Outside of mathematics, 'proof' is beyond scope.
What we can do is debate and discuss with the audience over the existence of detrimental and benefical actions, (in a social species setting), and which would be preferable to that social species. We don't need to invoke magic in order to do so.
What we do find however is that conclusions do indeed differ from culture to culture, person to person and era to era. The idea is to come to a workable agreement.
Theists have a problem with this because to them it isn't about discussion and agreement but unquestionable biblical mandate - hence why history has shown religion holding us back as far as moral improvement has been concerned.
religious morality, on the other hand, introduces god, who is inescapable, and whose rewards and punishments are irresistible, with those two values fixed to infinity, you have sealed your moral code, and you'd have any logical person unable to say he can logically ignore your moral code or not go by it
A theistic version of morality causes too many problems.
Firstly we must examine Euthyphro dilemma:
"Is what is morally good commanded by God because it is morally good, or is it morally good because it is commanded by God?"
If what is morally good is morally good because it is commanded by god then it is purely arbitrary. You'll hopefully see the problem if you pick the other option.
Furthermore, if we accept this objective morality we find ourselves in a further dilemma:
One day you are walking up a hill where you come across and old man attempting to slaughter a young boy with a knife. You know that such
action is immoral and so you stop him. It turns out however that this man is Abraham and god has ordered him to attempt to slaughter his own child. Unlike you, Abraham knows that his
action is entirely moral. Who is correct?
You can only say he is because
any command given by god
must be moral simply by the fact that he said it - even if you know the action is in fact immoral.
If god told you to rape your mother, raping your mother
must be absolutely moral given the fact that he's told you to do it. If you make the false statement that he can't because it's immoral, then you fall victim once more of Euthyphro. If god's commands are moral simply because he says them, then morality is completely arbitrary.
The theist has nowhere to go.