More than a century ago, in "The Brothers Karamazov" and other works, Dostoyevsky warned against the dangers of godless moral nihilism, arguing in essence that if God doesn't exist, then everything is permitted...
This argument couldn't have been more wrong: the lesson of today's terrorism is that if God exists, then everything, including blowing up thousands of innocent bystanders, is permitted — at least to those who claim to act directly on behalf of God, since, clearly, a direct link to God justifies the violation of any merely human constraints and considerations. In short, fundamentalists have become no different than the "godless" Stalinist Communists, to whom everything was permitted since they perceived themselves as direct instruments of their divinity, the Historical Necessity of Progress Toward Communism. --Slavoj Zizek, Defenders of the Faith, via ambivablog
Of all the arguments against atheism, the argument from morality must be the worst. I've tried to refute it several times on this blog, but Dr. Zizek nails it.
If God exists, than whatever God says, we must do. More precisely, whatever we believe God says, we must do. When Abraham heard a voice telling him to murder his son, he prepared to do so. When the Hebrews were commanded to conquer the nations of Canaan, they did so. When Islamic terrorists hear the voice of God telling them to blow up a pizza shop or fly into some skyscrapers, they obey. When Deanna Laney saw signs from God that she should kill her children, she did so.
After all, what does morality mean in a theistic worldview? If God commands it, it must be moral. That's objective morality.
The theists will surely argue that God didn't exactly tell Abraham to kill his child, that conquering the Canaanites was indeed moral, that the terrorists are fooling themselves, and that Mrs. Laney was surely insane. Well, where does that leave you? How do you know if your interpretation of God's will is correct, if you aren't fooling yourself, if you're sane?
Don't you have to see if your perception of God's will aligns with your own sense of morality before you can act on it?
Aren't you really in the same boat as us atheists? Except more in danger of being misled by voices, religious interpretations, or religious leaders?