The third point was his Manichaeism. The Persian religion was dualistic and produced, in the Hellenistic period, a movement called Manichaeism, from its leader Mani. It was a Hellenized Parsism, dualistic in character. So we can consider it a mixture between the prophecy of Zoroaster, the prophet of the Persian religion, and Platonism in the form of the Gnostic thinking of his time, the late ancient world.
These Manichaeans were for a long time the main competitors with Christianity.
They asserted that they represent the truly scientific theology of their time. Augustine was attracted for this reason and also because the dualism of the Manichaeans gave them the possibility of explaining sin rationally. This is the reason why the Manichaeans always had some influence through the whole history of Christianity. There were in the Middle Ages always sects influenced by Manichaean ideas, and there are Manichaean elements in many of you, without your knowledge of it. Whenever you hear an explanation of sin in terms of human freedom, then ask the question: “But if God is almighty, it must come from Him, or a principle against Him” – then you are Manichaean in your thinking: you have two principles in order to explain sin. This is something which is a past problem, but an actual problem, especially actual if you talk with people who are outside Christian: thought but have this popular nonsense with which they confront God’s almightiness and the evil of the world, and tell you either God is not almighty or He is not all-loving. Then you are tempted to retire into a kind of half-Manichaean principle that there is an ultimate principle of evil in the world against the ultimate principle of good. You hear this also unfortunately in very serious lectures, and when you hear that the Neoplatonists or Augustine called sin.”non-being,” then they have taken away the seriousness of sin. But in the moment in which you (regard) sin as a part of being, then you are Manichaean. .. Augustine was attracted ,by this because he could now have two ultimate principles – evil is as positive as good.
This choice, which kept him for ten years as a member of the Manichaean development, shows his interest in thinking. Not everybody had a merely logical interest in it. Most philosophers had other interests, too. There is first, that truth for this group, as for Augustine, is not a theoretical philosophical, it is not logical analysis, but is at the same time religious practice – practical truth, existential truth: that is his interest.