What’s the deal with this tension that we’re dealing with—one Paul arguably describes in Romans 7:19-20:
For I do not do the good I want, but the evil I do not want is what I keep on doing. Now if I do what I do not want, it is no longer I who do it, but sin that dwells within me.
and again in Galatians 5:17:
For the desires of the flesh are against the Spirit, and the desires of the Spirit are against the flesh, for these are opposed to each other, to keep you from doing the things you want to do.
At tymes the old self is in control, but at other tymes the new self is in the saddle; the struggle of life, according to this view is the struggle between these two aspects of the believer’s being. (Saved by Grace, 209)
While appealing, this “internal dualism”—where there are two persons at war with one another in the same person—doesn’t quite give us the best view of our ongoing struggle with sin.
Hoekema points out that Paul describes the “old self” as being definitively put to death on the cross—and in sanctification, we are progressively becoming more and more our “new selves.” Therefore, the believer who is easily discouraged by the continued persistence of sin (or the return of behaviors you thought you’d long since put to death), need not lose heart
A believer deeply conscious of his or her shortcomings does not need to say, Because I am still a sinner I cannot consider myself a new person. Rather, he or she should say, I am a new person, but I still have a lot of growing to do. (Saved by Grace, 213)
Do not be discouraged, Christian. The old self has indeed been put to death. We may have a lot of growing to do, but the new has surely come. Rejoice and do not lose heart.