In the 300s and 400s AD many Christians turned to believing in Manichaeism, a form of Christianity. A man named Mani started this movement in the late 200s AD. Mani lived in the Sassanian Empire, where lots of people were Zoroastrians. So Zoroastrianism influenced Mani’s thinking a lot. Manichaeans believed that the world was divided into the forces of Good and the forces of Evil. God was the leader of the good side and the Devil was the leader of the bad side. (Does this remind you of the plot of the movie Star Wars? Star Wars has strong Manichaean tendencies). This is a lot like the old Zoroastrian belief that the world is divided between the Truth and the Lie.
In 301 AD the Roman Emperor Diocletian started to persecute the Manichaeans. He succeeded in pretty much wiping out Manichaeaism in the Western Mediterranean and Europe. Some Manichaeans were killed. Many others moved to the Sassanian Empire. Others converted to Christianity. But even into the 400s AD there were a lot of Manichaeans among African Christians. The great African theologian Augustine, for instance, was a Manichaean first. He only later abandoned Manichaeism for Catholicism.
What’s wrong with Manichaeism? Why didn’t Catholics like it? Well, Catholics believed that there was only one God, who was all-powerful. If God and the Devil had the same amount of power, or even similar amounts of power, then there were really two gods, a good one and a bad one. If God was all-powerful, then why didn’t he just kill the Devil and get rid of evil in the world? People have been struggling with this question for a long time. Nobody has any definite answers. But Catholics felt that the answer could not be that there were two gods.
So thousands of Manichaean believers moved to the Sassanian Empire in the 300s AD. They kept their Manichaean beliefs there, and they convinced many more people to join them. So by the 600s when the Arabs conquered the Sassanian Empire, most ordinary people there were either Manichaeans or some other form of Christian. Gradually, over the next hundred years, most of these people converted to Islam.
But some of the Sassanian Manichaeans fled even further east into Central Asia. In Central Asia, they convinced the Uighurs to convert to Manichaeism, and many Chinese people as well. Most of these eastern Manichaeans were killed in the great religious persecution of the T’ang Dynasty, in the 840s AD. So Manichaeism started in Iran and spread east and west from there. Today hardly anyone would say they were Manichaean, but movies like Star Wars show that many people actually do think that God and the Devil, or the forces of good and the forces of evil, are fighting it out for your soul.