Manichaeans
In the 300's and 400's AD many Christians fell into the Manichaean heresy. This was started in the late 200's 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 400's AD a strong strain of Manichaeanism among some Christians. The great 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, 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.
