원죄
Augustine of Hippo (354–430) taught that Adam's sin is transmitted by concupiscence, or "hurtful desire", resulting in humanity becoming a "massa damnata" (mass of perdition, condemned crowd), with much enfeebled, though not destroyed, freedom of will. When Adam sinned, human nature was thenceforth transformed. Adam and Eve, via sexual reproduction, recreated human nature. Their descendants now live in sin, in the form of concupiscence, a term Augustine used in a metaphysical, not a psychological sense. Augustine insisted that concupiscence was not "a being" but a "bad quality", the privation of good or a wound. He admitted that sexual concupiscence ("libido") might have been present in the perfect human nature in paradise, and that only later it became disobedient to human will as a result of the first couple's disobedience to God's will in the original sin. 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. Justo Gonzalez interprets Augustine's teaching: humans are utterly depraved in nature and grace is irresistible, results in conversion, and leads to perseverance. Although earlier Christian authors taught the elements of physical death, moral weakness, and a sin propensity within original sin, Augustine was the first to add the concept of inherited guilt (reatus) from Adam whereby an infant was eternally damned at birth. Augustine held the traditional view that free will was weakened but not destroyed by original sin until he converted in 412 AD to the Stoic view that humanity had no free will except to sin as a result of his anti-Pelagian view of infant baptism.