Augustine was a disciple of Manichaeism until the age of 34 when he ‘converted’ to Christianity. The situation in which he left Manichaeism was due to his inability to obtain a position of power within the religion and remained an “auditor” the lowest level of follower despite obtaining the most highest academic achievement in Rome (professor of rhetoric in imperial court of Milan). For a man of such ego, knowledge and power to never attain any significant position or influence in the religion he devoted the last 17 years of his life too, made him angry. That is when he converted to Christianity, a fledgling religion which had significant depth of followers but a lack of unity in theological doctrine…
Now… what is a man with the the full range of knowledge of Manichaeism, who was a professor of rhetoric, master of neoplatonism to do? Well, let us look at what Manichaeism believes and cross it up with Neoplatonism and use Christian icons and archetypes and voila… Augustine Christianity. The serpent being Satan, Satan being the Fallen Lucifer, Fallen Cestial Beings, a battle between Good and Evil, etc the absolute freewill of man apart from God, eternal torment, many or all of these things were not Christian theology prior to Augustine but today is commonly taught as strong Christian doctrine.