Why did Will Durant say there was no historical example of a society holding on to moral life absent of religion? Did he miss any?

His assertion does not reflect the actual data. It depends. Some of the most atheistic countries in the world are Western Europe and certain countries in the far east--such as Japan. Some of the most religious countries in the world are in Africa and the Middle East. Which would you say is more moral? You have far less crime, less teen pregnancy, less conflict, less illiteracy, better healthcare, more freedom, more order, more happiness in the more atheistic countries.

The same holds true when you look at the United States. Those states with less religiosity fare much better than those with more religion. They perform better academically, economically, less crime, and the people are generally happier. Which is more moral?

When you look at the prison population, atheists make up only .07% of the prison population, yet in the general population atheists make up at least 3%. So why are atheists underrepresented in prison if we are so evil?

Finally, I would say religion, at least Christianity, gives license to immorality, since the doctrine of Grace says we are all damned to Hell, and the only way into Heaven is to believe in Jesus and repent your sins. Then, all your sins are forgiven and you go to Heaven, even if you're a murderer like Jeffrey Dahmer. So the mass murderer is forgiven his sins, and the skeptic, Carl Sagan, goes to Hell to burn for all eternity for the thought-crime of rational skepticism.

Christianity's doctrine of Grace makes Belief the only real virtue, and rational skepticism the only real sin. This is a prescription for gross immorality. It says a lot about humanity's innate morality that most Christians are decent people--they are decent in spite of their religion, not because of it.

He was limited by the thoughts of his times. The statement itself makes little sense.

He isn't very bright.

No