Why did the Roman Empire in the West fall apart in the fifth century? The argument started even before Odovacar forced the German puppet Romulus Augustulus, whimpering, off the stage in 476. When, in 410, Alaric and his Visigoths sacked Rome, old-fashioned pagans immediately blamed Christianity and the neglect of the old rituals for the catastrophe. Gibbon gave the pagan interpretation new life about the time of the American Revolution, and academic pagans ever since have either publicly or secretly endorsed at least part of his interpretation. Christianity, they whisper, is an Asian superstition that corrupted a healthy society. (What they are afraid to say but really mean is the equally false belief that Christianity is a Jewish cult.)
Christians were equally appalled by barbarian violence. The empire, so they believed, represented a natural and divine political order that had been corrupted by paganism. Purified since Constantine's time, the empire was now a bulwark of the Church. What did it mean when a horde of hoodlum savages could invade and sack the Eternal City?
In response to the sack, Saint Jerome lamented, "The human race is included in the ruins," and a shocked Saint Augustine wrote his masterpiece, The City of God. In what became one of the most influential Christian books of all time, Augustine explained that Christians, in abandoning the pagan gods, had not caused the destruction of...