Where does the old sin nature operate in a person?
The old sin nature operates in the physical body—the flesh and blood and bone that functions through the five senses. After Adam's original sin, the spirit, or place of fellowship, was made dormant, so fallen humanity lives in that same body, where the old personality and sin nature are active.
Les explains that ever since Adam acquired the original sin nature, the spirit, or the place of fellowship with God, was put out of commission and made dormant. So when people come into human experience, they live in the same body of flesh and blood and bone that operates the five senses. It is in that body that the old personality, the sin nature, is operating. Because of this, the lost person has no means of communication with God and is described as an enemy of God; even a cry for help is heard only when it is for salvation. Thus the old sin nature's sphere is the fleshly body, not a functioning spirit in fellowship with God.