On the latest Mere Fidelity episode, Derek, Matt, and I put on our theology nerd hats and discuss the concept of the felix culpa.
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Alastair's Adversaria 
I wonder if it will help this discussion if sin is not considered to be an existentially foreign substance (or non-substance) that emerges from parts of the Augustinian tradition, but rather a disordered relationship between things constituted as good. Thus the potentially non-Fall is in fact possible because nothing, not even the snake, doesn’t belong, but things may be out of place. God thus ordains the possibility of freedom for things to move from their rightful place, or rightful direction, towards a dis-placement resulting in dis-order, but that this dis-placement itself is not outside of scripted potentialities, which result in ultimate blessedness. God does not author sin, but He authors the result of sin, which is grace abounding even more.
I’m not too keen on the author-novel analogy, because it seems to suggest that God is in fact the author of sin, writing wicked characters to do wicked things.