Augustine of Hippo

Augustine is the deepest pool in Western thought after Plato. Confessions invented spiritual autobiography — nobody before him wrote like that about an inner life. City of God invented the Christian philosophy of history. His doctrines of original sin, grace, predestination, free will, the church, and the sacraments shaped everything Catholic and Protestant fought about a thousand years later. Calvin is downstream of Augustine. Aquinas is downstream of Augustine. Even modern atheist philosophers writing about selfhood and time keep returning to him.