Imagine a commuter finds a wallet stuffed with cash and no ID. Most people feel she should turn it in. But without God, what makes that should more than a preference, like liking coffee? If morality is just what societies agree on, it shifts with the majority. Slavery was legal; if human consensus is the final word, we can't say it was truly wrong, only unfashionable. The atheist can behave morally, sure, but that's borrowed capital from a moral framework they reject. God gives a standard above us and a final judge, so good and evil aren't just opinions. Without that, morality is possible as a feeling or custom. Not as a binding obligation. That's my case.