On October 28, 312 AD, Constantine defeated Maxentius at the Milvian Bridge north of Rome, ending the civil war between them and establishing Constantine as sole ruler of the Western Empire. The battle itself was not particularly difficult — Maxentius’s forces were pushed back onto the bridge over the Tiber, the bridge collapsed, and Maxentius drowned in the river — but what happened before the battle, or what Constantine subsequently claimed happened before it, transformed the event from a routine imperial civil war into one of the most consequential days in the history of Western civilization.

Rome was not original in its theology, and it did not pretend to be. The Romans were systematic borrowers of divine power, operating on the practical assumption that a god who worked was worth incorporating regardless of origin. The result was a pantheon that was Greek at its core, overlaid with indigenous Italian tradition, supplemented by imports from Egypt, Persia, and Syria, and eventually contested and replaced by a monotheism that originated in Judea. Roman religion was an accumulation, not a creation.