The view you describe is exactly the view that I take on the question of the Fall. I see no difference between what you say and what I believe about what happened in the Garden of Eden. That some sort of a “fall” occurred, is clear, as Paul and others talk often about the “flesh” or sinful nature. We are different because of the sin of Adam and Eve than we otherwise would have been. EXACTLY how we are different is not fully clear, but that we lost our innocence and that we gained a tendency toward sin is clear. What I do not believe is that we are born guilty of the sin of Adam. This is the classical doctrine known as “Original Sin.” The Original sin doctrine of Augustine, Luther, Calvin and Zwingli is a false one. These same leaders taught the doctrine of Total Depravity. This is the doctrine that we are born so completely depraved, that we are incapable of choosing to change our lives and to be saved. This leads to the classical predestination of Calvin, Zwingli and others. Again, this is false doctrine, as proved by so many clear and incontrovertible scriptures, such as 1 John 2:2, 1 Tim 2:4, 2 Cor 5:14 and many other passages.
You are right in accusing many believers of swinging the pendulum as an over-reaction to false doctrines. Many do indeed “throw the baby out with the bathwater.” I really hope that I am not guilty of this overreaction myself when it comes to the nature of the Fall of mankind. For what it is worth, it seems that you and I have landed in a very similar middle ground, which accepts that human nature was irrevocably damaged by the sin of Adam, but not that we are born guilty of Adam’s sin or completely depraved by inheriting that sin.
Augustine did not invent the concept of Original Sin. A less-well-developed version of this doctrine was used by Cyprian in the late third century. It appears that Augustine expanded the doctrine and made it more radical, especially as he included the idea of total depravity (which was not found in Cyprian). Yes, I agree that it is likely that Augustine shifted more to the “right” because of his opposition to Pelagius on the “left” of this issue. It is hard to prove the motivation of Augustine, so this is a bit of speculation on my part, but, yes, I believe that he reacted against Pelagius to make his doctrines of Original Sin and Total Depravity more radical. This is human nature, as you point out in your question. It is a thing that all believers must be careful to guard against in their own beliefs.
Pelagius was anathematized and Augustine was not for a very simple reason. The great majority of bishops, and especially the bishop of Rome supported Augustine. This does not prove that Augustine was right or that Pelagius was wrong, but the decision to issue an anathema was a political one within the church. It is a bit difficult to fully understand what Pelagius believed because the great majority we know of him comes from his fervent opponents. He clearly believed in free will and in the need for humans to decide to live a righteous life. He appeared to go too far, proposing that one could, by their own effort, stop sinning. This is called the doctrine of perfectionism. Again, it is hard to be sure how far Pelagius swung in this direction, as we have relatively few of his writings.
In any case, our beliefs must come from the Bible, and we need to be aware of our presuppositions, and our biases in order to have a correct biblical view. It is my opinion, from reading what you say above, that you have done a good job on this particular issue. I have only one suggestion. I suggest that you not call your view of the Fall “Original Sin.” To use this language is to tie yourself to the unbiblical idea that babies inherit the sin of Adam, which you properly reject because of Ezekiel 18. You and I believe that we inherit a propensity for sin, but are not born guilty of sin. I suggest you find a different vocabulary to describe your view of the Fall of Mankind.
John Oakes