Here’s Thomas Aquinas in Summa Contra Gentiles, Book IV, ch. 52:
“[We Christians] affirm that man was, from the beginning, so fashioned that as long as his reason was subject to God, not only would his lower powers serve him without hindrance; but there would be nothing in his body to lessen its subjection; since whatever was lacking in nature to bring this about God by His grace would supply.”
In other words, Aquinas is saying that the covenant (deal, bargain) God made with Adam and Eve was this: if they subjected their reason and will to God, then God would, by his grace, prevent their composite bodies (bodies consisting of parts) from disintegrating into corruption and death.
But Adam and Eve didn’t keep their side of the bargain, so God withdrew his grace. He let them age, corrupt, dissolve, die.
Put another way, the human body tends toward falling apart. Dissolution is the result of its potential as something composite reaching its actualization. In Thomism, this is called “potency” and “act”: all composite things posses potencies that are only sometimes actualized. Wood, for example, can become fire. It’s one of its potencies.
Likewise, if Adam and Eve had never sinned; if they had submitted their reason and will to God, then their bodies’ potential for corruption, by God’s grace, would never have manifested.
This means that if a lion had tried to eat Adam and Eve, or a storm to batter them, they would have gone on living by God’s ongoing miraculous protection and grace, but that was withdrawn after they stopped submitting their reason and will to God, and so their “potency” for corruption and death became their “actuality” (they really and truly aged and died). This withdrawal of God’s grace from them as composite beings has continued down to this day, to their descendants. We, Adam and Eve’s descendants, are being punished for our distant parents’ sin.
This is the doctrine of original sin.
Put another way, God used to miraculously hold off the body’s corruption and exposure to natural evil in Adam and Eve, even though that potential was there in them. God’s active grace was functioning in each moment of Adam and Eve’s existence. But after they sinned, that miraculous protection was withdrawn, and now we, their descendants, are left in the cosmos bereft to encounter the violence of the cosmos without God shielding us from it. We’re like Earth would be without its magnetic field deflecting the solar wind; we’re like the gray face of the moon, being worn down by time.
Yes, Thomas Aquinas was that crass; that medieval. We suffer, fall apart, and die because God withholds his grace from us. He could treat us better right now, in this very moment, but he is angry at the first parents, and his wrath is upon their descendants. We are children of wrath. Adam and Eve’s failure to submit their reason to God is why he now lets our composite bodies tend toward their potency into actuality. Through Adam and Eve, death entered the world, and we are suffering the consequences.
But what plausible higher good comes from this withdrawal of God’s protection? Does this sound like a sensible Higher Being providing a sufficient reason for why, for example, tsunamis have the power to kill hundreds of thousands of people at a single go? What’s the higher good coming from God’s ongoing grudge at the first couple’s sin?
Is God really so monstrous?
Prometheus Unbound 