If the devil were a broadcaster sending out his signal through the world, the sin nature in every person would be the receiver that is specifically tuned to welcome his message. The sin nature, sometimes called “the flesh” (Gal. 5:17, 19) or “old self” (Rom. 6:6; Col. 3:9), has a natural affinity for Satan’s values and his world-system. More so, the sin nature is not eradicated from the believer during his time on earth, nor is it ever reformed, as though it can be made to love God. What person can say, “I have cleansed my heart, I am pure from my sin”? (Prov. 20:9). No one is ever free from sin in this life, “Indeed, there is not a righteous man on earth who continually does good and who never sins” (Eccl. 7:20). Christians have a sin nature, and they do sin. Paul tells the Christians at Rome to “put on the Lord Jesus Christ, and make no provision for the flesh in regard to its lusts” (Rom 13:14). He writes to the Christians at Galatia and states, “But I say, walk by the Spirit, and you will not carry out the desire of the flesh. For the flesh sets its desire against the Spirit, and the Spirit against the flesh; for these are in opposition to one another, so that you may not do the things that you please” (Gal 5:16-17).
The Spirit and the flesh are in constant conflict. God could have removed the fleshly nature from believers at the time of their conversion, but He did not choose to do so. Why? He wanted to keep them continually reminded of their own weakness; to keep them continually dependent on Christ, their Priest and Advocate; and to cause them to praise unceasingly the One who saved such worms. Instead of removing the old nature, God gave us His own Holy Spirit to indwell us. God’s Spirit and our flesh are perpetually at war, and will continue to be at war until we are taken home to heaven. The believer’s part in the conflict is to yield to the Spirit.
The sin nature is resident in every person; both saved and unsaved, and is the source of internal temptation. “The flesh refers to that fallen nature that we were born with, that wants to control the body and the mind and make us disobey God.” Since the fall of Adam, every person is born with a sin nature, and it is this nature that internally motivates men to rebel against all legitimate forms of authority, both human and divine. At the moment of salvation, God the Holy Spirit indwells us and gives us a new nature that, for the first time in our lives, has the capacity and desire to obey God. Because the sin nature is not removed from the believer after salvation, the believer begins to experience conflict within. “The presence of two opposing natures (not two personalities) in one individual results in conflict.”
The old nature (which has its origin in our physical birth) fights against the new nature which we receive when we are born again (Gal. 5:16–26). No amount of self-discipline, no set of man-made rules and regulations, can control this old nature. Only the Holy Spirit of God can enable us to “put to death” the old nature (Rom. 8:12–13) and produce the Spirit’s fruit (Gal. 5:22–23) in us through the new nature.
Only the Christian has two opposing natures, and his spiritual growth guarantees internal conflict. The sin nature, though crippled at the moment of regeneration, does not give up control without a fight, and only the spiritually advancing Christian can overcome the power and habits of the flesh, as he devotes himself to learning and living Scripture by means of the filling of the Spirit. The Christian is to “lay aside the old self…and put on the new self which in the likeness of God has been created in righteousness and holiness of the truth” (Eph. 4:22, 24). Such language speaks to the reality that the believer has two natures, one that is corrupt and wants to please self and one that is new and wants to please God.
For the good that I want, I do not do, but I practice the very evil that I do not want. But if I am doing the very thing I do not want, I am no longer the one doing it, but [the] sin [nature] which dwells in me. I find then the principle that evil is present in me [the Christian], the one who wants to do good. For I joyfully concur with the law of God in the inner man [according to the new nature], but I see a different law [of sin] in the members of my body, waging war [causing conflict] against the law of my mind and making me a prisoner of the law of sin which is in my members. (Romans 7:19-23)
I have pondered Paul’s struggle between his two natures on many occasions. Every Christian who has grown spiritually and lived for any time in this world knows exactly what Paul is saying. He writes that he wants to do good, but then finds himself doing the very opposite. On the one hand Paul states that he “joyfully concur[s] with the law of God in the inner man” (Rom. 7:22), but that he experiences a war within because of “the law of sin” which is in his flesh (Rom. 7:23). This is the Christian experience.
Torn inside with desires to do that which we know is evil and new desires to please God, we experience the rage of the battle. The internal conflict manifests itself in everyday life as the believer is tempted to sin. The source of this conflict is the old sin nature, which is the root cause of the deeds of sin. In the conflict the believer is not passive. He has a vital role in determining to whom he will give allegiance—the old nature or the new nature. From the moment a sinner trusts Christ, there is a conflict in his very being between the powers of darkness and those of light. The one who has become a member of the family of God now faces conflicts and problems that he did not have before.
Though the Christian will struggle all his life with his two natures, he also knows the victory is already won. The sin nature has been defeated and its strength diminished because of the believer’s union with Christ (Rom. 6:6, 11). At his resurrection, the Christian is guaranteed a new body in heaven that is free from the sin nature as it will be just the like body of our Lord Jesus (Phil. 3:20-21).
Not only is the Christian commanded to “put on the Lord Jesus Christ,” but he is to “make no provision for the flesh in regard to its lusts” (Rom. 13:14). He must not only choose to live according to the new nature in conformity to the Spirit’s guiding, but must also learn to starve the monster that is his sin nature. To “make no provision for the flesh” means the Christian is to stop exposing himself to the things of the world that excite the flesh and lead to sinful behavior. The positive action is to grow spiritually with biblical teaching, Christian fellowship, worship and prayer so that the believer grows strong (Acts 2:42; Eph. 4:11-16; 2 Tim. 2:15; 3:16-17; 1 Pet. 2:2; 2 Pet. 3:18). It is only by spiritual growth and drawing closer to God that the Christian glorifies the Lord and learns to live in righteousness.
Modified excerpt from The Christian Life.
Dr. Steven R. Cook
 William MacDonald, Believer’s Bible Commentary: Old and New Testaments, ed. Arthur Farstad (Nashville: Thomas Nelson, 1995), 1893.
 Warren Wiersbe, The Bible Exposition Commentary: New Testament, Vol. 2 (Colorado Springs, Col., Victor Publishing, 2001), 18.
 Lewis S. Chafer, He that is Spiritual (Grand Rapids, Mich. Zondervan Publishing, 1967), 112.
 Warren Wiersbe, The Bible Exposition Commentary, New Testament, Vol. 2, 480.
 Robert P. Lightner, Handbook of Evangelical Theology (Grand Rapids, Mich., Kregel Publications, 1995), 206.