Augustine and the City of God
When Goths sacked Rome in 410, the shock was total — the city had not fallen to an enemy in eight hundred years — and pagans blamed the Christians for abandoning the old gods. From North Africa, Augustine answered with the City of God: the earthly city always falls, and always has; the city that matters is not made of walls. It let Christianity survive Rome's collapse by detaching itself from Rome's fortunes. His arguments on grace, war, time, and the self shaped Western thought so deeply that both Catholic and Protestant theology later claimed him.
A belief system rises, spreads, institutionalizes, and then schisms — changing the rules people accept as legitimate.