An important quote from "The Confessions of Saint Augustine" is, "It was a fiction based on my own wretched state, not the firm foundation of your bliss" (Book IV, Chapter 16). Much of "The Confessions" is devoted to Augustine's realization that the word of God was the only solid truth in the world. The "fiction" mentioned in this quote refers to the Manichean beliefs Augustine held prior to his conversion; the phrase "your bliss" refers to the Christian God's love.
Manichaeism is an ancient religion that originated in Persia and taught that the universe is gripped in an eternal struggle with the spiritual world of good and light against the material world of evil and darkness. During Augustine's time, Manichaeism was in competition with the Christian Church to replace people's traditional pagan beliefs.
It was a long and difficult process for Augustine to convert to Christianity. As he said in Book 6, "We did not relinquish our worldly aims, because we could not see the light of any truth that we might grasp in place of them" (Book VI, Chapter 10).
Christianity was a very demanding religion compared to paganism and Manichaeism. Yet, even though he was no longer sure of his belief in Mani, he was not quite prepared to renounce such earthly pleasures as sensual indulgence, pride and ambition because he did not know what, if anything, he would get in return. It took him years of further study before he could grasp the truth of teachings of the Bible.