Much of the Roman Pantheon was based off of the Greek ones. Like modern Europe, Greece was considered the primogenitor of civilization, thus were a highly insular society, holding the rest of the world, including their Roman masters, as unwashed brutes and barbarians. Now the Romans were highly tolerant of the religions of others, so much as A: they did not disrupt the public order and B: they offered praises to the Roman gods when mandated by the senate. Because the Greek gods were seen as mirrors to the Roman ones there was little religious strife between the Roman and Greek pagan religions. This is also why there was much friction between Judaism and the fledgling cult of Jesus Christ. A: Christians and Jews made general nuisances of themselves by decrying the debased nature of society, refusing to bathe, preaching in public, and practicing what, at the time, was an apocalyptic religion that urged people to abandon their families to join the cult. and B: the Abrahamic deity is a highly jealous deity so Jews and Christians refused to obey the state mandated sacraments to the Roman gods.

The first Christian cults started appearing in Greece around the turn of the 2nd century, with notable gatherings in Corinth and Thessalonica. There were also cults in Rome itself, Antioch, Jerusalem, and Alexandria. However, for most of the high Roman period, Christianity remained a curiosity. But the religion as a whole benefitted dramatically from the Greek neo-platonists, who saw the forward thinking ethical treatise of Jesus as well as the wholly dualistic nature of the universe as remarkably condusive to their philosophies, so the most important Christians of the day were the Greeks who refined Christianity into a much more philosophically sound religion than the folk-ish polytheisms of old.

Despite the dispersing of the Jews to all corners of the empire in the aftermath of the Jewish Wars, Judaism never caught on like Christianity did. The main difference being that Judaism does not place such a heavy emphasis on proslytization as Christianity does. Christians consider it a sign of glory and piety to bring another person into the flock, where as the Jews tend not to place such demands on its followers.

However, Judaism and Christianity were not the only Eastern Religions being imported into the Empire. The two most notable religions were the cult of Mithras and the cult of Sol Invictus. Both cults were monotheistic, held God’s power as absolute, and were expressly forbidden from the worship of other deities. Mithaism caught on particularly in the army, which is why Mithras came to be known as the soldier’s God, and Sol Invictus became the in vouge religion of the cosmopolitan Romans.

During the Crisis of the Third Century, the Empire fragmented into several kingdoms: the Gallic Empire in Hispania and Gaul, the old Roman Empire in Italy, Greece, Asia minor and Africa, and te Palmyrene empire in the middle east. It was only under the able administrations of the Illyrian Emperors that the Empire was reforged. One of the Illyrians, Emperor Aurelian, sought to strengthen the bonds of unity within the Empire so in 271 CE he made Sol Invictus the supreme deity of the Roman Pantheon, even above Jupiter/Zeus, and made it the official state religion. Greece at this time would have been a hodge-podge of religions, from classical paganism, to Sol worship, to Christians. Then in 312 CE, Emperor Constantine issued the edict of Milan, ending the persecution of Christians under the previous emperors and making Christianity an officially recognized cult of the Empire. It is speculated that this was done primarily for political purposes, because although only 10% of the Imperial population were Christian, virtually the entire army was Christian, having successfully driven Mithaism into obscurity. With his new found “faith”, Constantine sought to remake the empire in a new light by moving the capital from the current de facto capital of Mediolanum (modern day Milan) to Byzantium, which he christened “New Rome”. You’ve probably heard it refered to by its more used nickname which outlasted New Rome: Constantinople, the city of Constantine.

Emboldened Christians used this new found legal acceptance to make political strides, as late antiquity is hallmarked by deep religious strife between Christians and non-Christians or deviant Christians such as Arian Christians. Then, in 393 CE, Emperor Theodosius made Christianity the official state religion of Rome, and enacted a law making all other religions illegal. It was during these times that virtually all of the classical pagan institutions were burnt to the ground by zealous Christians. The Pantheon, for example, a masterpiece of architecture, was only barely saved at the last minute when it was converted from a pagan building into a Chruch. By the 5th century, only pockets of paganism remained, such as a few of the Roman aristocratic families and certain Greek pagan cults that had been driven underground. For all intents and purpose, Rome was a Christian nation by 400 CE.

Despite the numerous deviant Christians, even after the Nicene Creed codified trinitarian Christianity as the supreme religion, the Roman world maintained its Christian heritage by the handful of holy sees that were scattered through out the empire: the Holy See of Rome, Constantinople, Antioch, Jerusalem, and Alexandria. Each one maintained equally significant religious clout, but the Holy See of Rome and Constantinople became the two primary Sees of the world due to their position at the top of the political hegemony. The holy Sees of Antioch, Alexandria, and Jerusalem would fall in the late migration era with the rise of Islam, leaving the Church of Rome and the Church of Constantinople, who maintained a “separate but equal” relationship. However, tensions were inevitable, and tensions would reach a boiling point in 1054 CE, when the Pope of Rome and the Patriarch of Constantinople summarily excommunicated each other. Thus, the churches became officially split between the Catholics of Rome and the Orthodox of Constantinople.

Records indicate that small pockets of pagans in Greece remained until well into the middle ages.