The first problem that comes up with this question is one of semantics. For example, many prefer "sin nature," others prefer "sinful nature," and still others prefer the ambiguous "flesh." Whatever the specific names used for the warring parties, what is relevant is that an ongoing battle rages within the Christian.
The second problem is the actual definition of "nature." How this significant word is defined determines how one sees the distinction between the “old man” and the “new man” and its relevant outworking in the life of the Christian. One way to view "nature" is to understand it as a "capacity" within a believer. Thus, the old man is interpreted as the former way of life, that of an unbeliever. In this sense, the Christian has two competing capacities within him—the old capacity to sin and the new capacity to resist sinning. The unbeliever has no such competition within; he does not have the capacity for godliness because he has only the sin nature. That’s not to say he cannot do “good works,” but his motivation for those works is always tainted by his sinfulness. In addition, he cannot resist sinning because he doesn’t have the capacity to not sin.
The believer, on the other hand, has the capacity for godliness because the Spirit of God lives within him or her. He still has the capacity for sin as well, but he now has the ability to resist sin and, more importantly, the desire to resist and to live godly. When Christ was crucified, the old man was crucified with Him, resulting in the Christian’s no longer being a slave to sin (Romans 6:6). We “have been set free from sin and have become slaves to righteousness” (Romans 6:18).
At the moment of conversion, the Christian receives a new nature. It is instantaneous. Sanctification, on the other hand, is the process by which God develops our new nature, enabling us to grow into more holiness through time. This is a continuous process with many victories and defeats as the new nature battles with the “tent” in which it resides—the old man, old nature, flesh.
In Romans 7, Paul explains the battle that rages continually in even the most spiritually mature people. He laments that he does what he doesn’t want to do and, in fact, does the evil he detests. He says that is the result of “sin living in me” (Romans 7:20). He delights in God’s law according to his “inner being,” but he sees another law at work in “the members of my body, waging war against the law of my mind and making me a prisoner of the law of sin at work within my members” (v. 23). Here is the classic example of the two entities, whatever terms they may carry. The point is that the battle is real, and it is one Christians will wage throughout their lives.
This is why believers are encouraged to put to death the deeds of the body (Romans 8:13), to put to death that which makes a Christian sin (Colossians 3:5), and to put aside other sins such as anger, wrath, malice, etc. (Colossians 3:8). All this to say that the Christian has two natures—the old and the new—but the new nature needs continual renewing (Colossians 3:10). This renewing, of course, is a lifetime process for the Christian. Even though the battle against sin is constant, we are no longer under the control of sin (Romans 6:6). The believer is truly a “new creation” in Christ (2 Corinthians 5:17), and it is Christ who will ultimately “rescue [us] from this body of death. Thanks be to God—through Jesus Christ our Lord!” (Romans 7:24–25).