The material on the "sin nature" in this OP is provided by Kenneth Wuest (1893-1961), former professor of the "New Testament Greek," Moody Bible Institute.
It's vital that we grasp an understanding of the sin nature in order to build a foundation in which to approach this all important chapter 6 of Romans. This is what I will attempt to do in the OP, and hopefully, Lord willing, we can move on into the chapter with a better understanding of exactly what the great apostle is teaching us.
Mr. Wuest gives us the definition of the sin nature.
"The sin or evil nature is actually the Adamic nature which imprisoned man at the fall. It has poisoned the entirety of the human race and for all time. It is the nature which encourages sin and which against, man is powerless.
At Calvary Jesus broke the hold of this deadly yoke. However, He did allow it to remain, but powerless. Its remaining is a disciplinary measure. If the Believer correctly follows Christ, there is no problem; however, if we yield to temptation and sin, and then try to overcome in the flesh, the sin nature comes alive with serious consequences.
So, in this chapter we will study this all important subject of sin in the life of the Christian, why it is there, and the victory afforded by Christ -- the only victory there is incidentally."
The Christian and the Sin Nature
When the medical profession speaks of a disease germ that has not yet been isolated, it means that germ has never been identified and thus isolated from those germs which are known. Since that germ has never been identified, medicine has not been able to discover a remedy for it. Once the germ has been isolated, a remedy can usually be found. This is the case of the Believer.
The Christian who has never isolated the evil nature -- that is, who has not discovered the Truth of Romans Chapter 6 where God through the Apostle Paul describes the inner change which occurs at the moment he is saved, and also the Christians adjustment to this inner change, does not have consistent victory over it. But when in the Christian's thinking, this matter is cleared up and this nature isolated, he has the remedy which will enable him to gain consistent victory over sin in his life.
The Bible has thus isolated the germ called sin; identifying it as the fallen nature received of Adam. This nature remains in the individual even after God has saved him, as we learn from 1 John 1:8,
"If we say that we have no sin (sin nature), we deceive ourselves, and the truth is not in us."
God, in salvation breaks the power of this sinful nature over the Believer, but leaves it in him as a disciplinary measure. When the Believer refuses its behests, saying a complete "Yes" to Jesus Christ, he glorifies God, defeats Satan, and grows in spiritual strength and stature.
If the Believer expects to to gain consistent victory over this nature, He must know two things: A. What God has done in his inner being with regard to that nature; B. What adjustments it is necessary for him to make in relation to it. These two things Paul takes up in chapter 6, which we are now studying.
Sorry for the long post, just one more thing concerning the approach to Chapter 6, as we learn how to learn from the Apostle Paul. Mr. Wuest continues.
The first thing we must settle, regards the word "sin." Does it refer here to acts of sin committed by the Believer, or to the depraved nature (sin nature) still in him?
Inasmuch as the definite article appears before the word "sin" in the Greek Text, in other words, "the sin" this means we are referring here to the sin nature, actually to sin reigning as a king (Rom. 5:21).
Every time the word "sin" is used in this Chapter 6 as a noun, if the reader will substitute the words "sinful nature" or "sin nature" in its place, interpretation will be much easier.
Hopefully we can build a foundation on what Mr. Wuest has here given us. And If anyone would like to go into this Chapter 6 of Romans to understand exactly what Paul is saying about this sin nature, let's do it!