In the 300s and 400s AD many Christians turned to believing in Manichaeism, a form of Christianity. This was started in the late 200s AD by a man named Mani, who lived in the Sassanian Empire and was very much influenced by Zoroastrianism. Manichaeans believed that the world was divided into the forces of Good and the forces of Evil, and 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 began persecuting the Manichaeans and succeeded in pretty much wiping out Manichaeanism in the Western Mediterranean and Europe. Some Manichaeans were killed, and many others moved to the Sassanian Empire. Others converted to Christianity. But there remained into the 400s AD a strong strain of Manichaeanism among some African Christians. The great African theologian Augustine, for instance, was a Manichaean first, and only later abandoned Manichaeanism for Catholicism.
The problem with this idea, from the point of view of Catholics around the Mediterranean, was that there was only supposed to be 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, and don't have any definite answers, but Catholics knew that the answer could not be that there were two gods anyway.
When thousands of Manichaean believers moved to the Sassanian Empire in the 300s AD, they kept their Manichaean beliefs there, and indeed they convinced many more people to join them, so that 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 Christians converted to Islam.
Some of these Sassanian Manichaeans fled even further east into Central Asia, where 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.