No civilization on earth was ever established without a foundational religious tradition to create and maintain the morality that acts as the glue that binds individual people into a unified nation. (Try to think of one.) Furthermore, no civilization has ever outlived its founding religious traditions, although those traditions may progress and change over time. The religion may be refined and transformed like Christianity was during the reformation, but religion can never be fully repressed without dire consequences.
Perhaps the greatest “nonevent” in the history of Western civilization was the disappearance of traditional religion. Philosophers during the enlightenment that dominated intellectual thought beginning in the seventeenth century with Denis Diderot and reinforced in the eighteenth and nineteenth centuries with thinkers like David Hume, Voltaire and Johann Wolfgang von Goethe believed that mankind would eventually abandon religious belief as superstitious nonsense. They argued for a society based upon reason, as it was supposed to have existed in an idealized and largely mythical ancient Greece rather than faith and religious doctrine.
Of course, it never happened. Belief in religion is as strong today across all nations and in all societies as it ever was. In China, the Soviet Union, Cambodia and all nominally “socialist” regimes where religion is (or was) suppressed or even prohibited, religious worship remained strong in the underground, and flourished once those regimes were overthrown. A belief in Spirit is a universal human trait. God is NOT dead!
Secular law alone, without the help of a religious foundation, is no more stable than religion alone without secular law. The Soviet Union, Nazi Germany, Communist China under Mao and the Khmer Rouge of Cambodia are four modern examples of societies whose elites tried to do away with religion and replace it with a non-metaphysical “political theocracy”. These all ended in disaster, and hundreds of millions dead. Furthermore, even during the height of these regimes, traditional religions did not die out, but were practiced secretly. Modern Communist China is an example of a secular country where Christianity and Falun Gong, as well as a myriad of other religious beliefs flourish in spite of constant repression by the government.
Secular law is devoid of the human emotional element which can make people WANT to do things that they would rather not do. Secular law has no rational other than governmental expedience. Without a moral foundation to make people obey secular law, governments need very strong enforcement agencies to make the people obey.