In the book Confessions of Saint Augustine, by Garry Wills, why does Augustine lost faith in Manichaeism, a philosophy that divided the cosmos...

In order for us to understand why Augustine turned his back on Manichaeism, we need to understand why he followed that dogma in the first place, and the answer is really fairly simple: he didn’t believe in Christianity because he didn’t believe a compassionate and loving God could stand by and watch the world writhe in so much pain and suffering. The dualism of Manichaeism, in which the physical world was ruled by evil and the spiritual by good, made more sense to him. It also allowed Augustine to justify his debauched lifestyle: “man was powerless to overcome evil so long as he was held captive by the evil body” (Hart, 1988). But Augustine also had trouble reconciling the dualism of Manichaeism with all the beauty he saw in the world. If the physical world was inherently evil, why did he see so much good? This question eventually led to his rejecting Manichaeism.

It would be easy to simply say that Augustine then turned to Christianity, but his conversion was a long process that began with reading the works of Plato, one of whose central ideas is that the material world is simply a representation of reality. Although Augustine later rejected this notion, as well, reading Plato got Augustine thinking about the transcendent (Hart, 1988) and helped him make sense of some of the more ethereal ideologies in the Bible. Later, Augustine began to follow the words of Bishop Ambrose of Milan. Because Ambrose was a man of superior intellect, Augustine reasoned that Christianity must not be a religion for the ignorant, and he began studying the Bible. Augustine came to the conclusion that his lifestyle was not something over which he had no control but that it was caused by sin. However, he did not seem able to give it up. Although studying Scriptures and seeking God, Augustine continued to live a morally questionable life until one day, he heard a young girl singing what he interpreted as “’Tolle, lege. Tolle, lege [Take up and read. Take up and read]’” (Hart, 1988). He grabbed his Bible and it fell open to Romans 13:13-14, which commands that one behave properly by not drinking and having immoral sex but by “[putting] on the Lord Jesus Christ” (New International Version). At this point, Augustine immediately and completely renounced his sinful lifestyle and turned completely toward God.

As to how modern people make sense of Augustine’s tales of his sexual sin, there are two basic lenses through which we must examine this question. First, because of Augustine’s faith-based opinions on sexual sin, we must examine this question through a Christian lens. According to Hunter (2002), Augustine’s big mistake was in conflating sexual desire with sin, believing that

Adam and Eve's choice to disobey God had led to disobedience within their own bodies. Sexual desire, because it operates independently of the human mind and will, became for Augustine a privileged symptom of the sinful human attempt to assert autonomy against God. The result of the original sin, Augustine argued, was that human beings lost control even over themselves.

Hunter (2002) further states that “by the time he wrote the Confessions, Augustine viewed marriage as one acceptable solution to his problem with sexual desire. He presented marriage as a legitimate way to manage the difficulties presented by unrestrained desires.”

These views are not too different than one the Church espouses today. Rishmawy (2013), summarizing a conversation he had with Timothy Keller, a Christian apologist and founder of the Presbyterian Redeemer Church in NYC, argues, “Illicit sex is an idol in our generation” and “one of the biggest obstacles to repentance for revival in the Church is the basic fact that almost all singles outside the Church and a majority inside the Church are sleeping with each other.” He also cites research that shows perhaps as many as 90% of young people, including Evangelicals, are having “sex outside the bonds of marriage.” We can see in this one sources the focus on marriage as the ‘cure’ for sexual sin because it provides a proper place for desire to manifest. As we can also see, 

We do not know who God might be calling us to present with the Gospel’s call to sexual holiness. Keller’s challenge is for the Church to humbly but boldly call the Augustines sitting in our pews and local city coffee shops, bound fast in sexual sin, to turn and repent by the Spirit’s power to the true liberty of the Gospel. (Rishmawy, 2013).

For the Church, there is still a call to sexual holiness, to not be bound to and controlled by sexual desire. The converted and repentant Augustine would approve.

The second lens through which we must examine the question is, of course, a secular one. There is little research necessary here to draw a conclusion about how secularists and atheists would understand Augustine’s views on sexual sin: they are antiquated. They belong to a time that is past. Today the sentiments are “If it feels good, do it” and “Love is love.” There is little room for any idea of sexual purity, sexual sin, or controlling one’s desires. Although the converted and repentant Augustine would be grieved, the young one would wholeheartedly agree.