From Augustine, Manichaeism and the Good by Kam-lun E. Lee (dissertation for the Saint Paul University, 1996):

Augustine’s development of the idea of predestination reveals the Manichaean concept of the Good [the Summum Bonum] at work in three ways: on the framework of that development, in the implication of determinism, and on the context of the doctrine. To respond to the Manichaean view of the universe as a mixture of good and evil, Augustine suggests an alternative theory of cosmic ordering. Despite the presence of evil, he believes that the while cosmos is in harmonious beauty so long as evil is assigned to its proper place. God is to preserve this order in both the physical and the spiritual (moral) creations, an order portrayable with a two-tiered frame. Initially (around 388), Augustine thought that an individual person, as a spiritual creature, should have self-determination by the exercise of the will. But gradually, due to his conviction that personal evil is inevitable (a view shared by the Manichees and demonstrated in his conceptions of the consuetudo [the nature of humanity’s evil] and concupiscentia [desire or longing]), Augustine assigned determination of one’s destiny to the jurisdiction of God. As he neared the maturation of his predestinarian idea (around 396), therefore, Augustine increasingly subsumed the individual’s election or condemnation, which belongs to the moral order in the spiritual cosmos. Determinism, however, is not the only characteristic feature of Augustine’s version of predestination. The cosmological and eschatological contexts of his doctrine demand the notion of summum bonum to warrant the beauty of the cosmic order as well as to assure the elect’s eternal tranquil beatitude.

My notes are in brackets.

It is important to note that Augustine’s Platonism (specifically Augustine’s concept of the Summum Bonum) led to predestination as Calvinism knows it today. Augustine’s developments in theology were in contrast to and supplementing platonic and Manichean thought of his day. As noted before, Augustine had little patience with people who took the Bible seriously.