So what does the man who explained it actually say about Original Sin in his City of God? Thanks for asking.
Augustine is usually thought to have invented the idea, which has come to embody everything that’s wrong with religion to the modern mind: petty doctrines, pointless guilt, fear and mind control. Bad Augustine!
But of course, he didn’t really invent it, and it’s not what people think. He builds on Paul’s Romans 5:12f and 1 Corinthians 15, Irenaeus, his teacher Ambrose, Justin Martyr, Tertullian, Cyprian and others. Irenaeus — the church’s first professional heresy hunter — said Adam was a big baby whose fruit-eating fit made God mad but doesn’t seem to be genetic. “In the person of the first Adam we offended God, disobeying his precept” (Heresies V:xvi:3).
Our old friend Tertullian had more to say, inventing an idea called Traducianism, which claims human souls are physically located in our DNA and are inherited. (An opposing school that used to be called Creationism — not modern Creationism — held souls are injected by God at conception.) Adam’s sin corrupted his physical soul-body, and as all of our great-great-yadda-yadda-yadda-fathers, he literally passed it along to us, so that “that which is from God is rather obscured than extinguished” (De Anima 41).
Augustine also inherited the notion in ancient biology that all future offspring were microscopically present in their ancestors. So purely as a scientist, he believed that (a) all of us were microscopically present body and soul in our ancestor Adam, and (b) when Adam sinned we were all LITERALLY there!
Thus, we really are all guilty in some way and truly deserve God’s punishment: two kinds of death, physical and spiritual. “No one prior to Augustine had so thoroughly depicted the sinful complicity of all humanity with Adam,” says Daniel L. Akin in A Theology for the Church.
Now, all of this is old genetics and not that important to the Big Question: Why did Adam and Eve disobey God in the first place?
Augustine takes this head-on in Book XIII. Delicious.