AE 35:369: source of all sin, which is unbelief in the inmost heart. As, therefore, faith alone makes a person righteous, and brings the Spirit and pleasure in good outward works, so unbelief alone commits sin, and brings forth the flesh and pleasure in bad outward works, as happened to Adam and Eve in paradise, Genesis 3. Hence Christ calls unbelief the only sin, when he says in John 16[:8–9], “The Spirit will convince the world of sin … because they do not believe in me.” For this reason too, before good or bad works take place, as the good or bad fruits, there must first be in the heart faith or unbelief.[1]
“Our first parents were seduced through the serpent (Gen. 3.2). He was not just a natural serpent, but in him hid the serpent of hell who injected the lethal poison of sin in them. They spread that into all their descendants and into the entire human race. The expulsion of this poison and the healing of our corrupt nature did not require a common nature but the sort to be prepared for us from the divine and human natures in Christ…”
(Gerhard, On Christ, p.143).