98% of theists (don't ask where I got those statistics. I don't know.) I've talked to for some reason think this is an end-all good argument, when it really is a poor one.
They usually start out by asking if you believe in good and evil. My answer is yes, with a but. Yes but...people aren't all good or evil. Good and evil are very vague and broad terms, especially the word good. The idea of Good and Evil isn't the same for all people, but generally most people living in the same place agree on a few ground rules.
They usually disregard the last bit, as it doesn't fit into what I'm saying. "So you come up with what is good and evil? Then by who's standards do you decide what is right or wrong? Is Hitler evil? He didn't think so..."
We don't need anyone's standards. In the USA, at least, we are a democracy, we decide on what set of rules governs us. We vote on who we want to represent us and come up with the laws we follow. Not only that, every set of actions you can commit has a consequence which benefits you or society.
I heard this from a guy I now only consider a acquaintance ,"Cheating on your boyfriend isn't against the law! Would you do it if he'd never find out?"
No and no. Why would you do something "just because no one would find out." Nearly impossible. I'd feel guilty. And he wouldn't cheat on me and even if he did, two wrongs don't make a right. Is 'No one will find out' ever a good idea to do anything? It's a pretty stupid reason.
Then once they find out you have a conscience, they tell you some big man in the sky gave it to you.
1. What evidence do you have that this whinely little voice is actually an angel on my shoulder.
2. Wouldn't it interfere with free will? It does have a lot of say on what I do.
Next, I slap them with the same question they asked me, usually phrase a little different, but a lot meaner.
"So if God didn't exist... would you kill someone if you could get away with it?"
Most of them will say no. But the ones who say yes, the conversation ends there.
If they say no, why do you need a God in the first place to guide you morally? Are you really that morally stupid that you can't distinguish a good decision from a really bad one?