The Bible discusses the composition of man as three parts: a living soul [1 Cor. 15:45], body and spirit [1 Cor 2:11]. What happens in salvation is the new birth of the soul, mortification of the flesh and exchange of the spirit of man for the spirit of God.
The new birth of the soul means it comes alive to a relationship with the living God. Before the new birth it was dead to a relationship with the living God but now it is alive.
The mortification of the flesh is what Paul writes in Romans in terms of it being ‘buried in the baptism of death’ [Romans 6:4].
In this process you now have a soul alive unto God and the death of the flesh or old man. An inner conflict that did not exist prior to salvation now exists in the life of a man. Paul writes about the conflict in Romans chapters 6 – 8 with the ultimate exhortation to submit yourself to works of righteousness [Romans 6:15-23].
In Romans 7:14-25 Paul also writes more particular of the inner battle. Let’s see these verses for more insight:
“14 For we know that the Law is spiritual, but I am of flesh, sold into bondage to sin. 15 For what I am doing, I do not understand; for I am not practicing what I would like to do, but I am doing the very thing I hate. 16 But if I do the very thing I do not want to do, I agree with the Law, confessing that the Law is good. 17 So now, no longer am I the one doing it, but sin which dwells in me. 18 For I know that nothing good dwells in me, that is, in my flesh; for the willing is present in me, but the doing of the good is not. 19 For the good that I want, I do not do, but I practice the very evil that I do not want. 20 But if I am doing the very thing I do not want, I am no longer the one doing it, but sin which dwells in me.
21 I find then the principle that evil is present in me, the one who wants to do good. 22 For I joyfully concur with the law of God in the inner man, 23 but I see a different law in the members of my body, waging war against the law of my mind and making me a prisoner of the law of sin which is in my members. 24 Wretched man that I am! Who will set me free from the body of this death? 25 Thanks be to God through Jesus Christ our Lord! So then, on the one hand I myself with my mind am serving the law of God, but on the other, with my flesh the law of sin.”
The source of the conflict is the sin that continues to dwell in the old man. In salvation the old man is not converted but mortified yet it is still alive in that it is part of you. Therefore the sin within the old man, although dormant since you are no longer subject to the law, yet can be aroused by your actions. This is why Paul says in Romans Chapter 6 to not submit yourselves to unrighteous acts because not only is it sin but it will awaken the sin within you.
Paul writes in Galatians 5:16-18:
16 But I say, walk by the Spirit, and you will not carry out the desire of the flesh. 17 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. 18 But if you are led by the Spirit, you are not under the Law.
How do you ‘live by the Spirit’? The core actions you take are to read your Bible and pray and avoid your old life that stimulates the sin in your old man.
Bible study and prayer strengthen your soul or inner man and avoiding the actions, places and people that stimulate you to sin weakens the power of sin within your old man. Paul talks about this battle in Galatians 5:17, “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.”
How do you know if you are walking by the Spirit? Paul describes in Galatians 5:22-23a the fruits of living in the Spirt, “22 But the fruit of the Spirit is love, joy, peace, patience, kindness, goodness, faithfulness,23 gentleness, self-control…”
How do you know if you are not living in the Spirit? Paul describes these in Galatians 5:19-21a, “19 Now the deeds of the flesh are evident, which are: immorality, impurity, sensuality,20 idolatry, sorcery, enmities, strife, jealousy, outbursts of anger, disputes, dissensions, factions, 21 envying, drunkenness, carousing, and things like these…”
In conclusion Christian salvation brings about an inner fight between your inner man that is alive unto God and the old man where sin remains. How do you strengthen your inner man and keep the old man subdued? Read the Bible and pray daily and avoid the actions, people and places that stimulate the old man to sin.