Aurelius Augustinus, or Augustine of Hippo, or Saint Augustine is regarded as one of the most important figures in the development of Western Christianity. Thomas Cahill has called him the last Classical man and the first Medieval man.
For Roman Catholics and Anglicans he is called a saint and a pre-eminent theologian of the Church. Protestant churches regard Augustine as one of the fathers of the Reformation and his teaching heavily influenced many reformist churches such as the Calvinists. Some Eastern Orthodox Christian churches, however, have labeled him a heretic. 
Augustine was of Berber descent and was born in 354 in the Roman provincial city of Tagaste (in present-day Algeria). At age 17, he went to Carthage to continue his education in rhetoric. His mother, Monica, was a Berber and a devout Catholic, while his father, Patricius, was a pagan. To the despair of his mother, Augustine chose to follow the Manichaean religion of his instead of his mother’s.
Augustine lived a hedonistic lifestyle in Cathage, developed a relationship with a young woman who would be his concubine for over fifteen years, had a son and generally enjoyed a life of study and public speaking.
In 383, he moved to Rome, ostensibly to teach and practice rhetoric. He found the city to be less than he had hoped. Although he obtained a job as a professor, his students refused to pay their fees and Augustine used his connections to obtain a teaching position in Milan. It was, in fact, a considerable coup. At age 30, Augustine now occupied the most visible academic chair in Italy.
While his mother continued to press him to convert to Catholicism, the bishop of Milan, Ambrose, also drew Augustine away from Manichaeism. After a flirtation with skepticism, he became an enthusiastic student of Neo-platonism, and for a time believed he was making real progress in his quest to find the ultimate truth.
In 386, Augustine underwent a personal crisis and decided to convert to Christianity. He renounced his teaching position and gave up women and devoted himself to chastity and study. He claimed he heard the voice of child telling him to “take up and read” the Bible
Augustine would retell his spiritual journey in the book The Confessions, which became a classic of both Christian theology and world literature.
Augustine returned to North Africa where he founded a monastic order. In 391 he was ordained a priest and became a preacher (350 of his sermons remain.) He also became an ardent persecutor of Manichaeans.
In 396, he became bishop of Hippo. Augustine died in 430, at the age of 75, during the siege of Hippo by the Vandals. He is said to have encouraged its citizens to resist the attacks, primarily on the grounds that the Vandals adhered to the Arian heresy.
His works have been an influence on later writers, particularly in the ideas of fee will, the concept of original sin, and predestination. Augustine arguments were also used by the later church to sanction “just wars” and persecutions of witches and heretics.

The City of God

The City of God (De Civitate Dei) was written in Latin between  413 and 426. The treatise is an attempt to explain Christianity's relationship with competing religions and philosophies, and to the Roman government.
Rome was still in shock after having been sacked by the Visgoths and many Romans believed it was punishment for abandoning the old roman gods. Augustine attempts to rebuke that idea and say that a Christian city of God would ultimately triumph over Paganism
Augustine’s central message however was religious rather than political and he argued that Christians should be concerned not with earthly conflicts but with the mystical city of god (the New Jerusalem) rather than with Earthly politics. His theories went on to become central to the idea of the separation of church and state.
The book describes history as a conflict between the spiritual City of God and the earthly City of Man. The City of God is populated by people who forego earthly pleasure and dedicate themselves to Christian values, while The City of Man, consists of people who have strayed and rejected those beliefs.