Another example of the utter stupidity of the man. Rome had no such common religious values. Rome had all sort of different religious groups and sects. In fact the Romans BOASTED about worshiping all the gods.
The actual Spartan citizens were raised like worker bees. Called 'the Upbringing', it, too, was brutal. There was no individual freedom in Sparta. Athens was better, but most of the residents were not citizens with political rights, but resident aliens, or slaves. Think for yourself what this means for present day America.
I think you misread the quote. It doesn't say "a set of cultural values founded in a common religion." The VALUES are what bind, not necessarily the religion. If Religion A begets the same values as Religion B, then the culture has sufficient mortar to bind it.
At least that's my reading ...