Romans 7:18-25
(Rom 7:18 KJV)
For I know that in me (that is, in my flesh,) dwelleth no good thing: for to will is present with me; but how to perform that which is good I find not.

Here is the culprit that causes the Christian to sin. THE FLESH! Paul says that he wills to do good but that he cannot because sin abides in his flesh. Paul makes sure he emphasizes the fact that no good thing is what dwells in his flesh not his spirit. Romans 6:12 says, "Let not sin therefore reign in your mortal body, that ye should obey it in the lusts thereof." Do you see where the sin abides? It is in the flesh and not the spirit. There is a war waging between the mortal body and the immortal body which is our spirit essence. The real Paul, which is the redeemed Paul is not the one who is sinning because those that are born of God cannot sin.
(1 John 3:9 KJV) Whosoever is born of God doth not commit sin; for his seed remaineth in him: and he cannot sin, because he is born of God. In 1 John 3:9, we are told that those who are born of God cannot sin. The word “cannot” is a word of ability and not permission. The reason that those who are born of God cannot sin is because the soul has been redeemed and made perfect through the Lord Jesus Christ and the indwelling of the Holy Spirit. Therefore, the true believer in his spiritual essence cannot sin in his redeemed soul but because of the sinful flesh that we still abide in, we will sin, but those sins have already been paid for and removed through Calvary. (Col 2:13-14 KJV) And you, being dead in your sins and the uncircumcision of your flesh, hath he quickened together with him, having forgiven you all trespasses; {14} Blotting out the handwriting of ordinances that was against us, which was contrary to us, and took it out of the way, nailing it to his cross;

(Rom 7:19 KJV)
For the good that I would I do not: but the evil which I would not, that I do.

In this verse we see the struggle between spirit and flesh continue. It will never cease until we are freed from our earthly bodies. When we want to do good, we will do evil but the evil that we hate to do, we will do. When a Christian understands this principle that there is a sin struggle between the regenerated soul and the unregenerate flesh, there will be less spiritual depression when sin happens in the life of the believer because they will now understand what is happening and will be able to better prepare to prevent the flare up of sin in the flesh.

(Rom 7:20 KJV)
Now if I do that I would not, it is no more I that do it, but sin that dwelleth in me.

Paul reiterates that sin is the culprit and not "I." Since the sin nature is still remaining in the flesh, even after salvation, we will be able to understand the source of the sinful acts that we do. However, one must realize that because the sin nature is alive in the believer, does not give them permission to go and live a sinful life and then blame it on the sin nature. We must struggle against the sinful desires of the flesh to keep it under control. We are still responsible to live a holy life before the Lord.

(Rom 7:21 KJV)
I find then a law, that, when I would do good, evil is present with me.

Good is always hounded by evil. We saw this principle back in Genesis 4 with Cain.
(Gen 4:7 KJV) If thou doest well, shalt thou not be accepted? and if thou doest not well, sin lieth at the door. And unto thee shall be his desire, and thou shalt rule over him. God gave Cain this eye opening principle. Sin lies at the door and if we open that door we open that door to sin. Any time we want to do good, we must be on our guard because the sin nature can crop up at anytime and entangle us in sin.

(Rom 7:22 KJV)
For I delight in the law of God after the inward man:

Here is the verse that we need to look at as it ties together 1 John and Romans. The law of God after the inward man is a synonym for salvation, it is the regenerated spirit, in contrast to the outward man which is corrupted flesh. The reason Paul says that it is not "I" doing the sin is because he was given the same spirit that "cannot" sin as all believers are. The reason we have such a struggle between spirit and flesh is because we have a pure spirit given to us by God which is incapable of sinning and we have flesh that is corrupt and highly capable of sinning. As the war continues between a pure spirit and corrupt flesh we will always feel bad when we sin. This is how we grieve the Holy Spirit and sin should grieve us too. So to sum up this exegesis we may conclude that the passages in 1 John are teaching us that because we are born again of the Holy Spirit and since the Holy Spirit is eternal God, we, in our spirit essence cannot sin but since we live in the flesh that pure spirit is surrounded by corruptible flesh which continually seeks to sin. This why we always will have a spirit-flesh struggle.

Rom 7:23 (KJV)
But I see another law in my members, warring against the law of my mind, and bringing me into captivity to the law of sin which is in my members.

Paul saw and experienced the law that sin must be obeyed and the way that sin is obeyed is through the use of the members of the body. The law of practicing sin is warring against the law of the mind and that is the mind of Christ that every believer is given.
(1 Cor 2:16 KJV) For who hath known the mind of the Lord, that he may instruct him? But we have the mind of Christ. The mind of Christ in the believer is the result of salvation. So when we sin with our body it is waging war against the redeemed mind which we possess. Sometimes we get caught up in sin and it literally takes us captive as one who is a prisoner of war. Paul wants nothing more than to be freed from that law of sin which is still being obeyed by the unregenerate physical body. This should also be the same cry of the believers that we want to be free from the law of sin which is obeyed by our physical bodies.

Rom 7:24 (KJV)
O wretched man that I am! who shall deliver me from the body of this death?

In ancient Rome a punishment which was common was when a person killed somebody else, they would take the dead body of the murdered person and attach it to the murderer. Now after a while the rotting flesh of the corpse would begin to rot the flesh of the killer which would eventually kill him. Well this is the situation with sin, eventually it begins to eat away at us like a cancer in our body. The physical body which we possess is our body of death and that body actually hinders us from our full potential in serving the Lord.
(Mat 26:41 KJV) Watch and pray, that ye enter not into temptation: the spirit indeed is willing, but the flesh is weak. We must always be on guard because the law of sin has the ability to titillate this body of sin into sinning against God. Laws are made to be obeyed and that is how strong the law of sin is in our bodies.

Rom 7:25 (KJV)
I thank God through Jesus Christ our Lord. So then with the mind I myself serve the law of God; but with the flesh the law of sin.

Here is the great dividing line in the life of the believer. We serve with the regenerated mind, the law of God but the physical body we possess still serves the law of sin because it remains in its unregenerate state until death. So this is why Paul annunciates the fact that there are two camps in the body of the believer. There is the redeemed camp which is the regenerated soul and then there is the physical body which is the unregenerate part. It is the latter which gets us into trouble if we do not allow the spirit to rule over the flesh.