What is sin?

How do we account for the human propensity for evil? How is that propensity transmitted from one generation to the next?Liberal scholars and Jewish scholars typically say the Christian doctrine of original sin is based on the NT (Rom 5/1 Cor 15)  rather than the OT (Gen 3). 

Let's begin with a traditional exposition:

[Original] pollution is not  to be regarded as a substance infused into the human soul, nor a change of substance in the metaphysical sense of the word…Original sin is also an inherent positive disposition toward sin…[The unregenerate] cannot change his fundamental preference for sin and self to love for God… L. Berkhof, Systematic Theology (Eerdmans, 1984),246-46.

There is, of course, more to the traditional doctrine of original sin, but this is what I wish to focus on. Here's one possible way of viewing the human propensity for evil, and its transmission from generation to generation. Suppose humans naturally have a capacity for evil, and the effect of the Fall isn't to add or change something intrinsic to human nature, but to remove something extrinsic to human nature. To a great extent, sin is due to lack of moral inhibition. Suppose moral inhibition is initially due to divine restraint. If God withdraws the inhibitory constraints, then natural appetites take over. Competition for what we desire. Resentment for rivals. Rage when our desires are frustrated. 

In addition, the absence of moral restraint makes evil increasingly compulsive. The more they give into sin, the greater the appetite. The more hardened they become. 

We see the progression of sin up to the Flood. Likewise, in Rom 1, Paul takes about God giving people over to sin. That suggests the removal of divine restraint. 

Perhaps this is what is meant by the enigmatic "knowledge of good and evil" in Gen 2-3, which commentators puzzle over. It's possible to have an abstract knowledge of right and wrong. That, however, is different from having a sense of shame, obligation, or indignation. 

In that sense, a conscience can be acquired. Moral knowledge becomes internalized and increasingly engrained. Not nature, but second nature.

It requires external restraint for that to develop. If external restraint is withdrawn before the process develops, then a moral free fall ensues, because there's nothing to counteract natural appetites. Abstract moral knowledge is too weak. Moral formation is short-circuited.