Augustine of Hippo
One of the most important figures in the development of Western Christianity. In his early years he was heavily influenced by Manichaeism and afterwards by the Neo-Platonism of Plotinus, but after his conversion and baptism (387), he developed his own approach to philosophy and theology accommodating a variety of methods and different perspectives. 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 worshipped God.