This is all under grace and is not legalism or a works salvation. This was part of Paul's teaching to the saints in various congregations, though it is virtually unheard of in our day. Of course, the eternal security advocates will deny this, as they also deny any sin committed after regeneration will cause loss of salvation, but their denials won't change the facts any more than the serpent's denial of what the Sovereign God told Adam changed what happened when Adam and Eve sinned. You must overcome in this battle with sin or be lost eternally.
Therefore, brothers, we have an obligation - but it is not to the sinful nature, to live according to it. For if you live according to the sinful nature, you will die; but if by the Spirit you put to death the misdeeds of the body, you will live (Rom. 8:12,13)NOTE: A saint will either stay alive spiritually or die spiritually based on his choice to either start to live according to the sinful nature (flesh) or by the Spirit put to death the misdeeds of the body. The Christian must kill (put to death) the misdeeds of the body or they will spiritually kill him through sin. If you a Christian you MUST crucify the flesh.
Don't you know that when you offer yourselves to someone to obey him as slaves, you are slaves to the one whom you obedience - whether you are slaves to sin, which leads to death, or to obedience, which leads to righteousness? But thanks be to God that, though you used to be slaves to sin, you wholeheartedly obeyed the form of teaching to which you were entrusted. You have been set free from sin and have become slaves to righteousness. I put this in human terms because you are weak in your natural selves. Just as you used to offer the parts of your body in slavery to impurity and to ever-increasing wickedness, so now offer them in slavery to righteousness leading to holiness. (Rom 6:16-19)As sin leads to spiritual death, obedience leads to righteousness. Remember that.
And do this, understanding the present time. The hour has come for you to wake up from your slumber, because our salvation is nearer now than when we first believed. The night is nearly over; the day is almost here. So let us put aside the deeds of darkness and put on the armor of light. Let us behave decently, as in the daytime, not in orgies and drunkenness, not in sexual immorality and debauchery, not in dissension and jealousy. Rather, clothe yourselves with the Lord Jesus Christ, and do not think about how to gratify the desires of the sinful nature. (Rom 13:11-14)
Put to death, therefore, whatever belongs to your earthly nature: sexual immorality, impurity, lust, evil desires and greed, which is idolatry. Because of these, the wrath of God is coming. You used to walk in these ways, in the life you once lived. But now you must rid yourselves of all such things as these: anger, rage, malice, slander, and filthy language from your lips. Do not lie to each other, since you have taken off your old self with its practices and have put on the new self, which is being renewed in knowledge in the image of its Creator. (Col 3:5-10)As just stated, greed is a type of idolatry. (Read about the Gospel Of Greed.) In the above two passages we learned what is included in the sinful nature (Col. 3:5) and how to kill that beast (crucify the flesh). Not thinking about how to gratify the sinful nature but instead thinking about how to please God is the key (Rom. 13:14b). Remember that precious truth. This proves, as do other Scriptures, the vital importance of one's thought life. Ponder this:
Those who live according to the sinful nature have their minds set on what that nature desires; but those who live in accordance with the Spirit have their minds set on what the Spirit desires. The mind of sinful man is death, but the mind controlled by the Spirit is life and peace (Rom 8:5,6).That passage shows our behavior is the result of the thoughts we ponder either to gratify the flesh or the Spirit. (Change your thought life and your behavior will change.) That is how you crucify the flesh. This fact is one of the primary reasons why all Christians should be memorizing Scripture and talking about God and the things of God as often as possible throughout the day and night. See Psa. 119:9-11 and Deut. 6:6-9.
The thought life is also where the devil will tempt. (The devil will give us thoughts and imaginations, tempting us to do evil.) Paul took captive every thought:
We demolish arguments and every pretension that sets itself up against the knowledge of God, and we take captive every thought to make it obedient to Christ. (2 Cor 10:5)You must monitor your thoughts too. The devil will give you thoughts every day and sometimes those thoughts will be very subtle to slowly lure you away from God. You will need the whole armor of God to stand against his wiles (Eph. 6:13), but you can live holy.
So now you know how to put to death the sinful nature - that is, give no place to sinful thoughts. When temptations come, resist and reject them. Walk in the Spirit (ponder thoughts of how to please the Spirit of God) and you will not fulfil the lusts of the flesh (Gal. 5:16).
Fill your mind with God's truth and how to please Him. This is the victory in this battle to enter God's kingdom (to crucify the flesh). This is the all-encompassing way you can overcome evil with good (Rom. 12:21); and overcome you must or you will be thrown into the lake of fire (Rev. 21:7,8). What you do about this battle with your sinful nature and how it turns out will affect you throughout all eternity:
Do not be deceived: God cannot be mocked. A man reaps what he sows. The one who sows to please his sinful nature, from that nature will reap destruction; the one who sows to please the Spirit, from the Spirit will reap eternal life. Let us not become weary in doing good, for at the proper time we will reap a harvest if we do not give up. (Gal 6:7-9)Paul knew he must also win in his own personal spiritual conflict with the flesh or be lost eternally:
But I discipline my body and keep it under control, lest after preaching to others I myself should be disqualified (1 Cor. 9:27).Paul's repeated warning to the saints was about the need to crucify the flesh:
The acts of the sinful nature are obvious: sexual immorality, impurity and debauchery; idolatry and witchcraft; hatred, discord, jealousy, fits of rage, selfish ambition, dissensions, factions and envy; drunkenness, orgies, and the like. I warn you, as I did before, that those who live like this will not inherit the kingdom of God. (Gal 5:19-21)Again, a vital salvation truth is that we must crucify the flesh or die!