"Sin in the Scriptures means not only external works of the body but also all those movements within us which bestir themselves and move us to do the external works, namely, the depth of the heart with all its powers. Therefore the word do should refer to a person's completely falling into sin. No external work of sin happens, after all, unless a person commit himself to it completely, body and soul. In particular, the Scriptures see into the heart, to the root and main source of all sin: unbelief in the depth of the heart. Thus, even as faith alone makes just and brings the Spirit and the desire to do good external works, so it is only unbelief which sins and exalts the flesh and brings desire to do evil external works. That's what happened to Adam and Eve in Paradise (cf. Genesis 3)."
The root of all sin is - unbelief. So this got me thinking as I was driving home. Much talk surrounding sanctification is based upon outward acts. What one does. However, focusing one's view of sanctification upon what one does - upon the outward acts - is like focusing upon the outward acts of sin and ignoring the "movements within us." This is what we see in much "evangelical" theology. The denial of original sin leads one to focus only on the outward acts.