The Letters of St. Augustine by Saint Augustine
According to Wikipedia: "Augustine of Hippo (Latin: Aurelius Augustinus Hipponensis) (November 13, 354 - August 28, 430), Bishop of Hippo Regius, also known as St. Augustine or St. Austin, was a philosopher and theologian. Augustine, a Latin church father, is one of the most important figures in the development of Western Christianity. Augustine was heavily influenced by the Neo-Platonism of Plotinus. He framed the concepts of original sin and just war. When the Roman Empire in the West was starting to disintegrate, Augustine developed the concept of the Church as a spiritual City of God (in a book of the same name) distinct from the material City of Man. His thought profoundly influenced the medieval worldview. Augustine's City of God was closely identified with the church, and was the community which worshiped God. Augustine was born in the city of Thagaste, the present day Souk Ahras, Algeria, to a Catholic mother named Monica. He was educated in North Africa and resisted his mother's pleas to become Christian. Living as a pagan intellectual, he took a concubine and became a Manichean. Later he converted to the Catholic Church, became a bishop, and opposed heresies, such as the belief that people can have the ability to choose to be good to such a degree as to merit salvation without divine aid (Pelagianism). His works-including The Confessions, which is often called the first Western autobiography-are still read around the world."