Why can believers have comfort in the struggle with Satan and the old nature?
The ground of their comfort is that the One indwelling them is greater than the powers of Satan. Though old Adam causes failures and Satan works through the old nature, the Spirit and new nature oppose sin, and the indwelling presence gives assurance and strength for the struggle.
Les explains from Romans 7 that Paul describes the inner conflict: what he wants to do he does not, and what he hates he does. The old Adamic nature still slips in and gets the job done, and Satan works through that old nature as his playground, trying to get it to control and exercise its desires. Yet the new nature, prompted by the Holy Spirit, hates sin and tells him it is wrong. To control the failures of the body, that inner part must be strengthened and become uppermost in thoughts, acts, and deeds, constantly keeping old Adam in suppression. The devil is not often the direct cause; old Adam makes us do it, but Satan is not left out because he works through the old nature. Even so, believers have comfort because the One within them is greater than the powers of Satan.