Well, I don't know a good defense of the atheist standard of morality but I do know how to shut down religious claims that they have objective morality because of God.
They don't. It's the same subjective standards an atheist would use to come up with the rules society should follow. God says "these are my commandments" as if those are the standards we should follow. But then when he kills people, when he encourages slavery, rape, and lying, it's okay because he says so. When he throws a petty, jealousy fit or shows a toxic version of pride, it's okay because he's God. Christians in particular follow the morality that "might makes right" when they tout their God as being a source of "objective" morality.
They don't. It's the same subjective standards an atheist would use to come up with the rules society should follow. God says "these are my commandments" as if those are the standards we should follow. But then when he kills people, when he encourages slavery, rape, and lying, it's okay because he says so. When he throws a petty, jealousy fit or shows a toxic version of pride, it's okay because he's God. Christians in particular follow the morality that "might makes right" when they tout their God as being a source of "objective" morality.