Colin McGinn, responding to a question about whether morality needs a God, says the following (from here – I’ve transcribed it):
“Some people often think that if you’re an atheist, you don’t believe in God, then you can’t have a morality. There’s no foundation to morality and morality’s in question and so forth. […] It’s amazing to me that people in the current world still think that way, because that view was refuted 2.5 thousand years ago by Socrates, in the Euthyphro argument, where Socrates made the point that you can’t define goodness or rightness as what God commands, because the reason God commands it is that it’s right, it’s not right because God commands it. […] So, God cannot be the foundation of morality in that sense.”
This is good as far as it goes, perhaps, but in a Christian moral system (say), God isn’t just invoked as the revealer of the moral rules. What McGinn doesn’t address is why one would act according to the moral principles, and how an atheist can replace God in this sense.
It requires the following 2 things:
1. Rewards or punishments for following or not the moral rules.
2. A way to enforce the rewards or punishments.
This is because moral rules might conflict with self-interest. In response to the question: “Why should I do what is right?”, McGinn has no response (see here). In contrast, one of the principle functions of God in a Christian system is to enforce the moral rules (either in this world, or by sending someone to Heaven, Hell, or Purgatory).
If it were up to McGinn, it is as if it were sufficient for a society to just discover and write up good laws for regulating social conduct, but then not write up any punishments for disobeying the laws, and not have a justice system capable of enforcing the punishments!
(Also see here.)