Augustine's Evolving Thought on Indwelling Sin

In 359 Augustine acknowledges that a Christian still experiences the lusts of the flesh, but does not sin. At this point in his theology, he defines sin as the consent of the will to obey, or to act according to sinful desire. Simply having sinful desire is not personal sin... However, by the opening decades of the fifth century, Augustine's harmartiology expands. He begins to see sinful desire itself as personal sin and in need of the absolution brought about through the Lord's Prayer... While he only sees it as a venial sin and not mortal, it is still sin that a Christian must bear until the resurrection of the body. Augustine comes to see sinful desire as sin because it falls short of the perfect love of God and neighbor, which is the ultimate end of the law."

~Christopher T. Bounds, "Augustine's Interpretation of Romans 7:14–25, His Ordo Salutis and His Consistent Belief in a Christian's Victory Over Sin," The Asbury Journal 64, no. 2 [2009]: 24).