We don't need God to be good.
The objection, in full
Secular societies (Sweden, Denmark, Japan) score higher on every measure of human flourishing than religious ones. Empathy, evolutionary kinship, social contract — all explain morality without divine command. Many religious people behave terribly; many atheists are kind.
I don't believe in God and I'm not a murderer. So there.
or, in plain terms —I'm a good person without religion. End of story.
Behavior is not the question. Grounding is.
No one denies that atheists are often morally exemplary. The question is not who behaves morally. The question is what "morally" means — whether moral facts are real or merely useful fictions of evolutionary psychology.
If morality is a survival heuristic, it has no more authority over you than a heuristic about food preference. "You should not torture children for fun" becomes "primates of your lineage tend not to."
Most secular ethicists concede this and either bite the nihilist bullet (Mackie, error theory) or smuggle in a non-natural fact (Parfit, on the order of being). The smuggled fact is the interesting one.