Saint Augustine was one of the foremost philosopher-theologians of early Christianity and the leading figure in the church of North Africa. He had a profound influence on the subsequent development of Western thought and culture, and shaped the themes and defined the problems that have characterized the Western tradition of Christian theology. His two most celebrated writings are his semiautobiographical Confessions and City of God, a Christian vision of history.

Augustine was born at Thagaste in Numidia, which is part of present day Algeria. His father was a pagan (later converted to Christianity), but his mother was a devout Christian who labored untiringly for her son's conversion. As a child he was schooled in Latin literature and later went to Carthage to study rhetoric, where he became a teacher. By the age of twenty he turned away from his Christian upbringing. He was repelled by its codes of behavior, but he never completely renounced it.

At Carthage he became enthusiastic about philosophy after reading Cicero's 'Hortensius'. He considered becoming a Christian, but experimented with several philosophical systems before finally entering the church. For nine years, from 373 until 382, he adhered to Manichaeism, a Persian dualistic philosophy then widely current in the Western Roman Empire. With its fundamental principle of conflict between good and evil and its claim of a rational interpretation of Scripture, Manichaeism at first seemed to Augustine to correspond to experience and to furnish the most plausible hypothesis upon which to construct a philosophical and ethical system. Moreover, its moral code was not unpleasantly strict; Augustine later recorded in his Confessions: "Give me chastity and continence, but not just now." Disillusioned by the impossibility of reconciling certain contradictory Manichaeist doctrines, Augustine abandoned this philosophy and turned to skepticism.

About 383 Augustine left Carthage for Rome, but a year later he went on to Milan as a teacher of rhetoric. There he came under the influence of the philosophy of Neoplatonism and also met the bishop of Milan, St. Ambrose, then the most distinguished ecclesiastic in Italy. Augustine presently was attracted again to Christianity, and found Neoplatonism to be compatible with Christian beliefs. By the age of 32 he converted to Christianity and devoted the rest of his life to the pursuit of truth. Along with his natural son, he was baptized by Ambrose on Easter Eve in 387. He was ordained a priest in 391, and became bishop of Hippo Regius in 395. There he remained for 35 years as the intellectual leader of African Catholicism until he died on August 28, 430.

As a bishop Augustine had many debates with Donatists and followers of Pelagianism. Donatists were separatists who believed that they belonged to the only true church, and that sacraments were invalid unless administered by sinless ecclesiastics. Augustine replied that unity was the mark of true Christianity and that the sacraments depended on Christ and not on human institutions. Pelagianism was a reform movement led by a contemporary British monk who denied the doctrine of original sin. Pelagians believed that no one could be excused from not fully meeting God's law, stressing the importance of free will in controlling behavior. In the course of this conflict Augustine developed his doctrines of original sin and divine grace, which stood between the extremes of Pelagianism and Manichaeism. Against Pelagian doctrine, he held that because of original sin human nature was powerless to change; no one could be completely free to control one's motives without the gift of God's grace. Against Manichaeism he vigorously defended the place of free will in cooperation with grace.

Augustine's philosophy is always concrete, derived from his own personal experiences. For him Christianity is the true philosophy, Truth is one, and God is Truth. The possesion of Truth is happiness, and beatitude is the enjoyment of Truth. It is wisdom that gives knowledge of Truth, so the quest for Truth is a quest for wisdom. The sceptical Academics, who believed that wisdom consisted of knowing that we can know nothing, posed the question 'how does a man become wise' to Augustine, for to become wise one must desire the wisdom one lacks. But desire implies knowledge of the thing desired, so desire of wisdom implies lack of wisdom and possession of wisdom at the same time. Augustine answered in two ways; the first answer was, in Cartesian fashion, 'Si fallor, sum' (if I am wrong, I am), the second was from Isaiah 7:9, 'Unless you believe, you shall not understand.' Only faith provides the base from which the quest for wisdom starts, because faith is a knowing and a not knowing; it allows love of a thing known possible, and allows the desire to love the thing not yet enjoyed.

Some elements of Platonism can be seen in his De Trinitate. His view of the world is Platonic, there is the outer and the inner world, the lower and the higher, the sensible and the intelligible, and the carnal and the spiritual. To become wise requires a movement of the mind inwards and upwards to God, an opening of the mind to truth which is there provided the mental vision has been purified by faith. His theme of the divine in the world and in man is more biblical than Platonic, which allowed him to regard the material world with a reverence that would be impossible for a Platonist. His doctrine of evil as no-thing, a privation, is different from both Platonic thought and Manichaeism.

His thoughts on the Incarnation would upset Platonists. The divine image in man is defaced by sin, which upsets the divine order. It is restored by the Word which makes up for pride by humility, disobedience by obedience, restores life by enduring death, and innocence by taking the consequences of guilt. The Word incarnate is the Way back for man to the Word who is Truth, and the Way on to the risen Christ who is Life. Restoration comes from God's grace, and Divine grace is shown us in divine charity, and the human response is a response of charity. His ethics stems from grace before will-power, and from the personal relationship of love before abstract principle.

Augustine's success in unifying Christianity allowed it to become the religion of medieval Europe, and created a theology that has remained basic to Western Christianity, both Roman Catholic and Protestant, ever since.