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.

The unconquered sun — Sol Invictus — was the dominant religious force in the Roman Empire during the decades immediately before Christianity became the state religion, and the competition between them was closer than the outcome suggests. Aurelian, who reunified the empire after the chaos of the third century and who is one of the more underrated figures in Roman imperial history, established Sol Invictus as the supreme deity of the Roman state in 274 AD, built a spectacular temple in Rome, and created a new priesthood — the pontifices Solis — to administer its cult. For roughly forty years, the sun god was in a position of official supremacy that Christianity would not achieve until the reign of Theodosius nearly a century later.