The exchange life is the only way Paul teaches us in Romans to live as Christians. The life of Christ became our new life when we first accepted Christ. Our old adamic or sinful nature was crucified with Christ. We became new creations (2 Cor. 5:17) having received and now possessing Christ.
Unfortunately, we are new spirit beings still living in this old unregenerate body, or as Paul puts it, this body of death. Sin’s power over us was broken according to Romans 6:6 and that we have been set free from its power (6:6,7,and 11). But according to Romans 7:17,18,19,20,21 Sin is very much alive and still within our bodies. It is this “Sin” which controls the old man or what we call the old nature.
The life of Christ has replaced this controlling factor in us. According to Colossians 3:4 Christ has become our life. We were never created to be independent. Adam (us included) was created to be motivated and controlled by God. This does not mean that we were to be puppets but we were to have our natures derived from God Himself. Satan exchanged God’s control for his. The system he introduced to control us was the evil power called Sin. Satan through Sin controlled man, so man’s nature became sinful. Man’s nature became identified with its controller. All that was Satan’s became man’s. (Read Ephesians)
Now because of the exchange (being born again of the Spirit) we Christians are identified with Christ and no longer with sin. His life becomes our controlling factor. His Life becomes our new identity, His nature our nature. All that is Christ has become ours. We are now to yield ourselves to Him (Romans 6:13) and He will live His Life in and through us (Galatians 2:20).
But, according to Paul, if we yield to Sin, we will fall back into his control and find ourselves under his mastery (Romans 6:16). This is the reason the just shall live by faith. Because as long as we trust Christ to live for us, in us, through us, we will not sin or fall under Sin’s control. Walking in the Spirit or by the Spirit (Galatians 5:16, Romans 8:4) is our yielding to God.
There are two ways of falling under Sin’s control. First, the flesh, the cravings of our bodies outside of God’s control, may fall to Sin’s control by simply falling to one of his constant temptations. Since Sin was past on to us through our father’s seed, he has been with us all our lives and knows us in detail. Sin knows how to tempt us and with what to tempt us with, and He is very good at it. But remember, we are to yield to God and not to Sin according to Romans 6:12,13. Yes, he may tempt us but he is powerless to force his will over us because of our co-crucifixion with Christ. According to Romans 6 we are freed from the power that sin has over us.
The second is trying to live by the law. The law was given to expose sin. Christ is given to us to life out the law through us. It is indeed to be an unexplainable, miraculous life. For what the law could not do, in that it was weak through the flesh, God sending his own Son in the likeness of sinful flesh and for sin, condemned sin in the flesh, that the righteousness of the law might be fulfilled in us who walk not after the flesh, but after the Spirit. Romans 8:3,4