It may be helpful to look specifically at the terms "old man" and "new man" that Paul uses. They are unique to the writing of Paul, and I believe that a misunderstanding of these terms may hinder our walk in the life of God. Some translations substitute the term "sinful nature" for "old man," but this is an interpretation that can mislead our thinking. For our discussion, we will keep to the literal translation "old man" and "new man." I think it will help us to see more clearly what Paul is saying.
In Romans 5:12-20, Paul compares Adam and Christ, saying that Adam is a "type" of Christ, the new Adam who was to come. "Nevertheless death reigned from Adam to Moses, even over those who had not sinned according to the likeness of the transgression of Adam, who is a type of Him who was to come" (Romans 5:14). He goes on to compare the differences between Adam and Christ, referring to them as "one man" and "one Man." He continues in chapter 6, referring to Adam as "our old man." "Knowing this, that our old man was crucified with Him, that the body of sin might be done away with, that we should no longer be slaves of sin" (Romans 6:6). Notice that Paul uses another term here to refer to the race that descended from Adam, namely the "body of sin." The reference here is not to the physical body of one man, but rather to the whole "body" of Adam's race.
We find Paul using similar terminology in his letters to the Ephesians and the Colossians. "That you put off, concerning your former conduct, the old man which grows corrupt according to the deceitful lusts, and be renewed in the spirit of your mind, and that you put on the new man which was created according to God, in true righteousness and holiness" (Ephesians 4:22-24). "Do not lie to one another, since you have put off the old man with his deeds, and have put on the new man who is renewed in knowledge according to the image of Him who created him, where there is neither Greek nor Jew, circumcised nor uncircumcised, barbarian, Scythian, slave nor free, but Christ is all and in all" (Colossians 3:9-11).
There is one more place Paul uses the terminology "new man," and that is to describe how God has made both Jew and Gentile "one new man" in Christ Jesus. "For He Himself is our peace, who has made both one, and has broken down the middle wall of separation, having abolished in His flesh the enmity, that is, the law of commandments contained in ordinances, so as to create in Himself one new man from the two, thus making peace, and that He might reconcile them both to God in one body through the cross, thereby putting to death the enmity. And He came and preached peace to you who were afar off and to those who were near" (Ephesians 2:14-17).
Thus, according to Paul, God looks at mankind through two men, namely Adam and Christ. "And so it is written, 'The first man Adam became a living being.' The last Adam became a life-giving spirit" (1 Corinthians 15:45). The race descended from Adam is spoken of as "one man," "the old man," "the body of sin," "the body of death," and "a living being." The race descended from Christ, on the other hand, is referred to as "one Man," "the new man," and a "life-giving spirit."
The significance of this to us is that we began in the old man and have been transferred to (or born into) the new man through the provision of God at Calvary. The old man was not in us--rather, we were in the old man. There are many today who think of the old man being inside them, warring against the new man. They have been taught that this war will never cease while we are here on earth, and that they have to live with it until they get to heaven where God will give them a new body. I do not believe an honest reading of the scripture supports that view. This view leaves us in defeat and struggle, while Paul says that we are "more than conquerors" through Christ. We need not wait until heaven to be delivered from the old man. We are delivered when we are baptized into Christ with the Spirit. We are now in the new man who is created by God in true righteousness (Ephesians 4:24).
If we do experience conflict within, and it is not the "old man fighting with the new man," what is it? I believe there are several possibilities, and if we come before the Lord with an honest desire for the truth, He is faithful to show us where we are and give us a place to respond in faith, and to lift us up to victory.
The first possibility is that we have never been taken out of the old man and put into the new man. We may have come to Jesus in some manner, but we find ourselves in the place Paul once was: trying by his own efforts to do the will of God. We have good desires and intentions, but no power to obey. Paul describes this condition in Romans 7. Many today believe Romans 7 to be a description of the normal Christian life, but it is not. It is the normal struggle of a religious person trying to serve God by his own strength. Christ died to deliver us from this bondage! "O wretched man that I am! Who will deliver me from this body of death?" (Romans 7:24) How will we be delivered? "For the law of the Spirit of life in Christ Jesus has made me free from the law of sin and death. For what the law could not do in that it was weak through the flesh, God did by sending His own Son in the likeness of sinful flesh, on account of sin: He condemned sin in the flesh, that the righteous requirement of the law might be fulfilled in us who do not walk according to the flesh but according to the Spirit" (Romans 8:2-4).
A second possibility is that we have indeed been taken out of the old man and baptized into the new man, but we are trying to work out our relationship with God by self-effort and following rules, rather than drawing upon His grace in a faith relationship. This can produce the same bondage we just described. God has called us to walk by faith day by day, abiding in Him. If we have been made alive in Christ Jesus, the apostle Paul instructs us as follows: "As you therefore have received Christ Jesus the Lord, so walk in Him, rooted and built up in Him and established in the faith, as you have been taught, abounding in it with thanksgiving. Beware lest anyone cheat you through philosophy and empty deceit, according to the tradition of men, according to the basic principles of the world, and not according to Christ. For in Him dwells all the fullness of the Godhead bodily; and you are complete in Him, who is the head of all principality and power" (Colossians 2:6-10).
A third possibility is that we are interpreting the "war" within us wrongly. We all have human bodies that were exercised in sin before we came to Christ. Now we are being exercised in righteousness by the Holy Spirit. Change does not often come instantly or easily. It takes diligent obedience over a period of time to see our bodies brought under the control of the Spirit in all areas. Thus, Paul admonishes us to "put off the old man with his deeds" and "put on the new man who is renewed." To the Colossians he explains that since we have put off the old man and have put on the new man, we should not let the deeds of the old man manifest themselves, but instead put on the deeds of the new man. His basis of appeal for new conduct is that a believer is a new creation in Christ Jesus. The believer has a grace he did not have before, because of his standing in Christ Jesus. Paul admonished the Romans to "put to death the deeds of the body." We can obey Christ our Lord. A Christian can stop sinning. Not only can he stop sinning, but he must stop sinning if he wants to know fellowship with God (see 1 John 3:6-9). As we exercise our human natures in righteousness and obedience, the "war" will subside and we will find ourselves at peace with God and within ourselves.