I don't mean to be pedantic, but I'm not aware of *any* society that "lasts." They all seem to have a life cycle that is accelerated or retarded by certain moral aspects of their existence. What are the other two legs you are refering to?
It seems to me, according to my own studies, that Judaeo-Christian beliefs were intertwined with Natural Law to bring a touch of Spiritual Law, from above, for the individual and the society's benefit.
The 'law of the soul' was left to the individual's own responsibility by basing the Constitution in both as a societal guide to protect and assist the individual in their own responsibility.
Free will, but with the advantage of being a society that rewards righteousness but only demands limits on behavior.
Does that make any sense?