David, this is news to me, so I’d like to hear more. Obviously Augustinians may hold a differently nuanced view than Augustine himself, but my understanding is that he did not limit Adam’s effect on us to inheritance. Instead, we participated with Adam because we were in Adam in seed form. Yes, we inherit a corrupted sin nature from Adam, but we also participated in the sin that spawned this nature.
In City of God, he writes, “we all existed in that one man, since, taken together, we were the one man who fell into sin.” Even though “the specific form by which each of us was to live was not yet created and assigned, our nature was already present in the seed from which we were to spring” (13.14).
Similarly, in On Marriage and Concupiscence he states, “By the evil will of that one man all sinned in him, since all were that one man, from whom, therefore, they individually derived original sin” (2.15). And in A Treatise on the Merits and Forgiveness of Sins, and on the Baptism of Infants (MFS) he makes the connection repeatedly. “Adam is the only one in whom all have sinned” (1.19); “none whatever […] die except in Adam, in whom all sinned” (1.55).
In A Treatise Against Two Letters of the Pelagians, he writes that there are two viable options on how to read the last clause of Romans 5:12, but both reveal the same truth: “Let them, then, choose which they will,—for either in that ‘man’ all have sinned, and it is so said because when he sinned all were in him; or in that ‘sin’ all have sinned, because that was the doing of all in general which all those who were born would have to derive” (4.7). He says much the same thing two more times in the rest of the chapter, and also explains these two readings in MFS 1.11. Adam’s sin, according to Augustine, was the doing of all (and incidentally, he did lean heavily on Romans 5:12 to make his case, even if the surrounding verses and a few other passages were sometimes also mentioned).
And how, exactly, did Augustine picture us existing in that one man? The closest he gets to explaining his reasoning comes from his last work, the Unfinished Work in Answer to Julian (I’m quoting the 1999 Teske translation, though second-hand):
Some sort of invisible and intangible power is located in the secrets of nature where the natural laws of propagation are concealed, and on account of this power as many as were going to be able to be begotten from that one man by the succession of generations are certainly not untruthfully said to have been in the loins of the father. They were there […] though unknowingly and unwillingly, because they did not yet exist as persons who could have known and willed this (6.22)
We were all present in Adam in seed form, we participated in the sin with Adam, and the sin corrupted our seed form so that, together with Augustine’s view of concupiscence in sexual union (which I won’t get into now), we are conceived in sin. It isn’t just the sexual act that makes us conceived in sin, but this act confirms the sin we already participated in and bear the stain of.
Anyway, that’s how I’ve understood Augustine. I am quite interested in how he can be read as teaching that “Adam’s sin is his alone and is not in our debit column.”