During his early years as a student and teacher of rhetoric at Carthage and Rome, he was a confirmed Manichaean (a Persian religion, followers of Manes 216-277) but later in Milan began to study Neoplatonic philosophy and eventually converted to Christianity.
Just as Thomas Aquinas adopted the thought of Aristotle as the rational basis of religion, so did Augustine with the teachings of Plato and Platonism.
In his proof for the existence of God, he began with the fact that we are capable of acquiring mathematical knowledge and as Plato demonstrated, this awareness transcends the sensory realm of appearances.
He further argued that the eternal existence of numbers and their mathematical relationships required some form of metaphysical support. This support must generate from a greater being as the eternal source of the reality of these things. This being must be God.
At the heart of Augustine's philosophy was "Credo ut intellegiam," meaning "I believe in order that I may understand". He felt that only through faith could wisdom be obtained.
He saw both philosophy and religion as quests for the same thing - truth; however, the philosopher without faith could never attain the ultimate truth. Human reason and philosophy were only useful to those who had faith.
As a Christian theologian, Augustine remains famous for his agnostic contributions to Western philosophy. He rejected the epistemological criticisms by skeptics that there is no basis for claiming knowledge of our existence. In a proof similar to one later made famous by Descartes, he pointed out: "Si fallor, sum" "Even if I am mistaken, I am".
In other words, it's possible to employ the human faculties of reason and logic to pursue significant knowledge of the world.
Augustine is probably best known for his Confessiones (Confessions), which is a personal account of his earlier life, and for De civitate Dei (The City of God) which deals with issues about God, martyrdom and other Christian philosophies.
His early works, including writings on human will and ethics, had an influence on philosophers such as Schopenhauer and Nietzsche.
Wittgenstein was fascinated with how Augustine described the learning of language in Confessions and in his own work, Philosophical Investigations, successfully repudiated it.