Roman tradition dated the city's founding to 753 BCE, when Romulus was said to have drawn its sacred boundary, the pomerium, on the Palatine Hill after killing his twin brother Remus in a dispute over the site, a story Romans themselves treated as legend rather than history. What is certain archaeologically is that Iron Age villages on the Palatine and neighboring hills merged into a single city by the sixth century BCE, ruled first by kings and then, from 509 BCE, by a Republic whose magistrates held priestly office as a matter of course. The Capitoline Triad, Jupiter, Juno and Minerva, worshipped together in a great temple on the Capitoline Hill, stood at the center of that state religion, and the college of pontiffs, headed by the Pontifex Maximus, regulated the calendar and the ritual on which Rome's relationship with its gods depended.
As Rome's power grew from city-state to empire, so did its pantheon, absorbing the gods of conquered peoples and, from the first century CE onward, hosting foreign cults, Egyptian Isis, Persian-derived Mithras and eventually Christianity. Successive emperors, most severely Nero, Decius and Diocletian, persecuted Christians as a threat to the state cult that guaranteed Rome's prosperity, executions Roman tradition later located at identifiable sites in the city. Constantine's Edict of Milan in 313 CE ended that persecution, and by the century's end Christianity had displaced the old state religion entirely, leaving the title Pontifex Maximus, once held by the emperor, to pass by tradition into the vocabulary of the papacy. Mary Beard, John North and Simon Price's Religions of Rome remains the standard modern account of that civic religion's rise and eclipse.