Such religious promiscuity was quite routine in the ancient world. Indeed, no one did syncretism like Rome itself, its polytheistic system combining Roman, Greek, and Egyptian deities with aplomb.
Before Christmas, though, the Roman pantheon always won.
In the case of Christmas, we have the reverse—a religious festival from the hinterlands that ended up taking over Rome itself. This is the amazing thing about Christmas: the way it absorbed and triumphed over the paganism and imperial ideology that, the night Christ was born, was the only imaginative game in town.
The Roman world at the turn of the eras, after all, had many reasons of its own to celebrate. N. T. Wright observes in his new book Paul and the Faithfulness of God that the era of Augustus was the first time in the ancient world that history had a plot. Virgil and Horace put the old tales of gods, arms, and men into a single story. That story, it turned out, led up to the glorious present moment. An incipient "age of gold" was coming into being, under the beneficent rule of the man born Octavian and renamed Augustus Caesar upon his accession to uncontested power in 27 BC.
Like all Roman leaders, Augustus was ambitious, capricious and cruel on the way to the top, but once he had secured the sole rule of the empire, he also brought a certain amount of virtue and wisdom, along with stability and longevity, to his reign. The empire pulled out all the cultural stops, Wright observes, "from tiny coins to the rebuilding of entire city centres," to signal that "Augustus's rise to power was the great new moment for which Rome, and indeed the whole world, had been waiting."
The Augustan golden age was thoroughly religious as well as political. As the republic became an empire, its emperors edged closer and closer to divinity. Octavian styled himself "Augustus," a title just as august in Latin as it sounds in English. His adoptive father, Julius Caesar, had been posthumously deified in 42 BC, allowing the new Caesar to also style himself "Divi Filius," "son of the divine"—just one tiny step from "Dei Filius," "son of god."
Within one hundred years emperors like Caligula and Domitian would be appropriating divine titles while still living. (Not all were willing to do so. The more circumspect Vespasian quipped as he died in AD 79, "puto deus fio," "I think I am becoming a god.") The world of gods and men—or at least gods and emperors—always permeable, had now fused. Some of the most enthusiastic deification of emperors happened not in Rome itself, but in its colonial outposts. For the empire's most loyal subjects—or those who most needed to be seen as loyal—the Roman empire, with its peace and goodwill to all on whom the emperor's favor rested, was the goal toward which both history and religion had been moving all along.
The same era that brought the apotheosis of emperors was the very high water mark of polytheism and traditional Roman religion. Never had the Roman gods seemed so propitious, never had their temples been so lavishly ornamented, never had the military might that they blessed seemed so effective. Within a century of Jesus' birth, meanwhile, the capital city of the stubbornly monotheistic Jews would be reduced to rubble, leading a Jewish aristocrat like Josephus to conclude that the Jewish God, too, was on the side of the Roman Empire. Rome's paganism, with its patchwork of gods from every region it had come to dominate, was both creatively inclusive and utterly totalitarian in its insistence that all religious roads led to Rome.