Living Under the Shadow of a Dark Power
That dark power is most obviously sin, but for Augustine, that basically means anything that isn't God: lust, Manichees, pride because he wins speech competitions, the theater, etc.
Every Little Thing, Is Gonna Be Alright
Augustine is seemingly happy. He's on his hunt for truth, he thinks he's hanging with the right crowd. No problems here.
Um, Actually, No, It's Not
Augustine starts to realize that he really isn't happy, and that what he once thought were the answers are not really answers at all. Christianity, which seemed useless and nonsensical before, is suddenly looking shiny and full of truth.
Will the Dark Power Win?
Even though Augustine wants to love God, he can't imagine actually giving up all of the things he would need to give up (read: sex) to live the life of a true Christian.
Miraculous Redemption Is Miraculous
And we do mean miraculous. Right when Augustine is on the brink of conversion, he hears a child's voice telling him to pick up a book and read. What Augustine reads then convinces him that he needs to live a Christian life. The rest is history.