Why do some question the use of the expression, the believers two natures?
Sometimes defining terms helps resolve differences. At issue is what constitutes a nature? If we accept that a nature is the essential characteristics and qualities of a person or thing, we can suggest three possible objections.
A first objection is that this expression is not biblical. Some point out that we have become "partakers of the divine nature" (2 Peter 1:4). This cannot mean we have been made to share in the "essential characteristics" of deity, because those characteristics are non-transferable. If this referred to a "new nature," we would hardly have expected the word "partakers." Peter would more likely state that we received it or that God imparted it, since "nature" seems to imply something that has now become native to a believer. The suggested meaning is that, as possessors of new life - eternal life, we share the quality of life possessed by God.
A second objection is that it is difficult to define any "entity" or nature imparted at salvation. What was imparted to us at salvation is a new principle and power. We have been made alive spiritually and have received eternal life. Our quickened spirit or mind now has a relationship with God and delights in righteousness (Romans 7:22, 23). Because of this relationship, a new law or principle is now active in the believer.
In addition, the Spirit of God now resides within us. He empowers the believer to live righteously. This is a more intimate and effective provision than Old Testament believers enjoyed, yet they lived righteously as a result of having been made spiritually alive. Our minds have always been able to make moral choices, but when made alive spiritually our mind functions properly by responding to God and to righteousness.
A third objection is that "the essential characteristics and qualities of a person7 define that person. Is a believer, then, "defined" by "our old man" (Romans 6:6), his old standing in Adam, or by "the new man" (Colossians 3:10), his new standing in Christ? The essential characteristics of a believer in Christ are "righteousness and true holiness" (Ephesians 4:24). Every believer is a new creation (2 Corinthians 5:17) and this defines his characteristics.
Many able teachers and commentators refer to the believer's two natures, so it is best not to crusade on this point.
What causes the conflict within a believer?
The conflict often attributed to "the believer's two natures" is between the flesh and the Spirit (Gal 5:17). In Romans 7:23, the conflict is because the "law in my members" wars against "the law of my mind." The principle (or law) affecting the believer's mind delights in God's law (verse 22). The principle "in my members" is "the flesh," which invariably tends toward sin and, for all Adam's fallen posterity, is endemic to a body of flesh. Apart from redemption, this "law of sin which is in my members" is inseparable from a body of flesh; for a believer this link will be severed at the rapture. The "mind of the flesh" (Romans 8:7, JND) controls an unbeliever's spirit and soul; it will characterize him forever.
Galatians 5:17 accords with Romans 8:1-4 in teaching that the transforming power in a believer's life is the indwelling Spirit (Romans 8:9). The Spirit, Who jealously desires that we should be all for God (James 4:5), cannot peacefully coexist with the flesh. The flesh will never submit to God or delight in righteousness. The conflict will never cease while we are physically in the flesh - in a body of flesh. Because of redemption, we are no longer "in the flesh" morally (Romans 8:9). The controlling factor in our life is now the Spirit of God; we are "in the Spirit" (Romans 8:9). As we yield to the Spirit, righteousness will be expressed in our life. Until the Rapture, as long as we are in a body of flesh, however, the flesh will continue to resist this work of the Spirit.
What does "whosoever is born of God doth not commit sin" mean?
In his first epistle, John refutes false teaching about the distinction between the spiritual and physical realms. By classifying everything physical as sinful, false teachers dissociated the believer's body from his spirit. This implies that as long as a believer's spirit is enlightened, his bodily behavior is irrelevant. John contends that our relationship to God requires behavior consistent with God's revealed character.
Some have thought that I John 3:9 should read, "Whatsoever is born of God doth not commit sin," meaning that what God has produced in us (a nature?) is incapable of sinning. This interpretation would play into the hands of John's protagonists. That would indicate either that a believer is an enlightened spirit without sin in a body that sins or that his spirit doesn't practice sin, but his body does. John is teaching that the believer is one entity; his spirit and body are not independent parts. The believer is "born of God."
The key word is "commit," meaning a child of God cannot practicing sinning. Sinning cannot characterize one who is born of God because God is light. The epistle teaches that God's family will display likeness to the Father. A believer - not merely his body - still has the capability of sinning (1:8) and commits acts of sin (1:10); sin cannot, however, characterize him internally or externally.