원죄
Augustine of Hippo taught that original sin was both an act of foolishness ("insipientia") and of pride and disobedience to the God of Adam and Eve. He thought it was a most subtle job to discern what came first: self-centeredness or failure in seeing truth. The sin would not have taken place, if satan hadn't sown into their senses "the root of evil" ("radix Mali"). The sin of Adam and Eve wounded their nature, affecting human intelligence and will, as well as affections and desires, including sexual desire. The consequences of the fall were transmitted to their descendants in the form of "concupiscence", which is a metaphysical term, and not a psychological one. Thomas Aquinas explained Augustine's doctrine pointing out that the "libido" ("concupiscence"), which makes the original sin pass from parents to children, is not a "libido actualis", i.e. sexual lust, but "libido habitualis", i.e. a wound of the whole of human nature. Augustine insisted that concupiscence was not "a being" but "bad quality", the privation of good or a wound. The bishop of Hippo admitted that sexual concupiscence ("libido") might have been present in the perfect human nature in the paradise, and that only later it had become disobedient to human will as a result of the first couple's disobedience to God's will in the original sin. The original sin have made humanity a "massa damnata" (mass of perdition, condemned crowd). In Augustine's view (termed "Realism"), all of humanity was really present in Adam when he sinned, and therefore all have sinned. Original sin, according to Augustine, consists of the guilt of Adam which all humans inherit. As sinners, humans are utterly depraved in nature, lack the freedom to do good, and cannot respond to the will of God without divine grace. Grace is irresistible, results in conversion, and leads to perseverance.