The Problem of Reconciliation
From the view point of the Christian religion, of course, the primary struggle within human nature first appears as the struggle between the individual and God. This makes perfect sense once we understand human nature in relation to the purpose for us in Creation, as explained earlier. We were made able to serve God in freedom, but we rebelled and in rebellion we used our independent power (in our body) against God.
Upon conversion, however, we have peace with God (Rom. 5:1). But the problem of reconciliation then shifts to the self and to those two components the New Testament refers to as "the flesh" and "the spirit." How? After conversion our will and conscious intent are for God or "the spiritual," as we've seen with Simon Peter. But the layer upon layer of life experience that is embedded in our bodies, as living organisms born and bred in a world set against or without God, doesn't directly and immediately follow the shift of our conscious will. It largely retains the tendencies in which it has so long lived.
Source: Dallas Willard, The Spirit Of The Disciplines, pp.85-86.