Augustine did not arrive at his mature theology by a straight road, and the road explains the shape the theology took. As a young man in Roman North Africa he spent about nine years as a follower of Manichaeism, a dualist religion teaching that light and darkness are two eternal, warring substances and that evil is a real force rather than an absence of good. He left it unconvinced by its astronomy and its answers, passed through a period of Academic skepticism that left him doubting whether certainty was possible at all, and arrived in Milan still searching.
There, as his Confessions recount, reading certain books of the Platonists, almost certainly Plotinus and Porphyry in the Latin translation of Marius Victorinus, gave him what Manichaeism never had: a way to conceive of God as immaterial rather than a being extended in space, and evil not as a rival substance but as a privation, a falling away from the good rather than a thing in its own right. That reframing, together with Ambrose of Milan's preaching and a conversion experience in a garden in 386, carried him into Christian baptism the following year.
The two controversies that followed gave Augustinianism its lasting center of gravity, and both were arguments Augustine was uniquely positioned to have. Against the Manichaean view he had once held, he insisted evil was privation, not substance, safeguarding a wholly good creator. Against Pelagius, who taught that human beings can choose the good by will alone, Augustine, remembering how little his own will had accomplished unaided, argued that grace is not a reward for effort but its precondition. Augustinianism's theology of grace was forged, in large part, against the two errors he had spent his own life exhausting first.