St. Augustine was born in 354 in Roman-controlled Northern Africa. Born of a Catholic mother and a Pagan father, he studied Cicero and other pagan writers in his youth. He converted to Manichaeism in Carthage in 371; taking a concubine and fathering a son with her. He later developed and interest in Neoplatonism and then during a spiritual crisis in 386, converted to Catholicism, becoming a celibate priest. After this, he quickly ascended up the church hierarchy until he was made the Bishop of Hippo in 395, where he stayed until his death in 430.
The Roman Empire fell in 410, only seventeen years after Christianity became the official religion of the state. Many of the traditional Roman worshipers saw this as a causal connection, and began to attack Christianity. St Augustine, determined to defend Christianity, put forth his vision of what an empire built on peace and justice would consist of in his twenty-two volumes of the City of God. In the book, St. Augustine argues that an Emperor rules best and happiest when he is ruling by God’s will. This work would serve as the basis for the work of future religious political thinkers, such as John of Salisbury, who would expand upon the work to argue for the direct rule of religious authority over secular government.