Final answer:
Augustine joined the Manichean sect after their promise of truth and happiness, but later realized their promises were empty and found fulfillment in his Christian faith.
Step-by-step explanation:
As Bishop of Hippo, Augustine wrote against the Manichean heresy, which held that the universe was a battlefield between the forces of good and evil. The Manicheans promised Augustine that if he joined their sect, he would find truth and happiness. However, after joining, Augustine realized that their promises were empty and that the Manicheans were unable to provide the answers he was seeking. Instead, Augustine found fulfillment in his Christian faith and ultimately rejected the Manichean beliefs.