1 Thes 4:3-6 ESV (Emphasis added)
3 For this is the will of God, your sanctification: that you abstain from sexual immorality; 4 that each one of you know how to control his own body in holiness and honor, 5 not in the passion of lust like the Gentiles who do not know God; 6 that no one transgress and wrong his brother in this matter, because the Lord is an avenger in all these things, as we told you beforehand and solemnly warned you.
Now why would Paul, the great "Apostle of grace", solemnly warn Christians that they should be holy and sanctified by the Spirit in their conduct if they were already perfectly so? It makes no sense at all.
You see there is an element of sanctification that depends upon our actions. What we do and don't do. Paul wrote in Galatians 5, that if we walk by the Spirit, we will not fulfill the desires of the flesh. Those who do not walk by the Spirit do live in the flesh and will not inherit the Kingdom of God. That is his solemn warning. That even those who have believed can fall away from their inheritance if they live sensual and fleshly lives.
Galatians 5:16-26 ESV
16 But I say, walk by the Spirit, and you will not gratify the desires of the flesh. 17 For the desires of the flesh are against the Spirit, and the desires of the Spirit are against the flesh, for these are opposed to each other, to keep you from doing the things you want to do. 18 But if you are led by the Spirit, you are not under the law. 19 Now the works of the flesh are evident: sexual immorality, impurity, sensuality, 20 idolatry, sorcery, enmity, strife, jealousy, fits of anger, rivalries, dissensions, divisions, 21 envy, drunkenness, orgies, and things like these. I warn you, as I warned you before, that those who do such things WILL NOT inherit the kingdom of God. 22 But the fruit of the Spirit is love, joy, peace, patience, kindness, goodness, faithfulness, 23 gentleness, self-control; against such things there is no law. 24 And those who belong to Christ Jesus have crucified the flesh with its passions and desires. 25 If we live by the Spirit, let us also keep in step with the Spirit. 26 Let us not become conceited, provoking one another, envying one another.
Now, where does this battle begin? It begins in the mind!
Paul wrote to believers constantly to think on things of the Spirit.
Romans 8:5 ESV
For those who live according to the flesh set their minds on the things of the flesh, but those who live according to the Spirit set their minds on the things of the Spirit.
Philippians 3:18-19 ESV
18 For many walk, of whom I often told you, and now tell you even weeping, that they are enemies of the cross of Christ, 19 whose end is destruction, whose god is their appetite, and whose glory is in their shame, who set their minds on earthly things.
Colossians 3:2 ESV
Set your minds on things that are above, not on things that are on earth.
Philippians 4:8 ESV
Finally, brothers, whatever is true, whatever is honorable, whatever is just, whatever is pure, whatever is lovely, whatever is commendable, if there is any excellence, if there is anything worthy of praise, think about these things.
Romans 12:2 ESV
Do not be conformed to this world, but be transformed by the renewal of your mind, that by testing you may discern what is the will of God, what is good and acceptable and perfect.
Ephesians 4:22-24 ESV
22 to put off your old self, which belongs to your former manner of life and is corrupt through deceitful desires, 23 and to be renewed in the spirit of your minds, 24 and to put on the new self, created after the likeness of God in true righteousness and holiness.
Even Jesus chimed in on the subject:
Matthew 16:23 ESV
But he turned and said to Peter, "Get behind me, Satan! You are a hindrance to me. For you are not setting your mind on the things of God, but on the things of man."
So you see, there is more to the Christian life than just believing and doing whatever you please because sin is no longer an issue. True, we are all to crucify the old nature. Put it to death. Dead things do not move or breath or live (obviously) unless they are resurrected. So leave that dead man in the ground. Set your minds on things of the Spirit, walk by the Spirit, and so fulfill the will and purpose of God in your life!
1 Thess 4:1
For you know with a positive assurance what charges we gave you through the Lord Jesus, for this is the will of God, your being set apart [from sin to holiness], that you should hold yourselves off from fornication; that each one of you should know that he is to procure his own vessel [wife] in [personal] holiness and honor,
(from The New Testament: An Expanded Translation by Kenneth S. Wuest Copyright © 1961 by Wm. B. Eerdmans Publishing Co. All rights reserved.)
this is where 'spiritual dyslexia' has afflicted many Christians; as they see Gal 5:16 like this:
Gal 5:16 KJ(Traditions of man)V
(16) This I say then, do not fulfil the lust of the flesh, and ye shall walk in the Spirit.
Gal 5:16 KJV
(16) This I say then, Walk in the Spirit, and ye shall not fulfil the lust of the flesh.
walking in the Spirit refers to being led by God; and since He will never lead us to 'sin' (dwell on and/or commit transgressions of His laws), to the extent we are led by Him we will not 'sin'.
'Adultery, fornication, uncleanness, lasciviousness, Idolatry, witchcraft, hatred, variance, emulations, wrath, strife, seditions, heresies, Envyings, murders, drunkenness, revellings, and such like' are all indicators that we are not being led by God; and if anyone thinks that they are free of ALL of these things, they should think again...
ultimately, idolatry encompass ALL transgressions of the law; as idolatry is simply elevating one's own reasoning above God's (or said another way, respecting anything above God). no one in a flesh body on the earth today is in a perfect state of complete mind renewal by the Word of God; there are areas in every person's life that are not yet yielded to the Spirit of God. someone cuts you off in traffic and you stifle thoughts and words of wicked intent toward them rather than wonder what burden they're under to make them so careless - that's idolatry. you see a really good-looking person of the opposite sex and you stifle thoughts of fornication, but only after you entertain them for just a few seconds - that's idolatry. you see your coworker get the promotion you know you deserve when they should have been fired instead and you gnash your teeth to keep the words of cursing stifled - that's idolatry. you see a preacher driving a rolls royce and think about the toyota you drive and criticize him for receiving a better gift from God - that's idolatry. the drunk that got saved at church two weeks ago receives his complete and miraculous healing of liver cancer and you think about the pain in your back that you've been believing God for deliverance from for years and it really bothers you for some time that the drunk got healed - that's idolatry.
Rom 2:1 KJV
(1) Therefore thou art inexcusable, O man, whosoever thou art that judgest: for wherein thou judgest another, thou condemnest thyself; for thou that judgest doest the same things.
this isn't referring to wrong actions being judged; this is referring to judging the intent or condition of someone else's heart intentions; such as when that person cuts you off in traffic, etc.
yes; we are to become more and more holy and sin less as we mature as Christians - but this is the fruit of being led by God, not the root.
so then what Paul must be referring to in Gal 5:21 is not an occasional error, or a part of one's mind that is not yet fully renewed that results in an instance of idolatry; but a heart desire to live in complete idolatry that is evidenced by these ungodly acts he lists - in other words, those who have a heart desire to live in complete idolatry are not reborn in Christ, and these ungodly acts are indicators, not the cause of such a person's failure to inherit the Kingdom of God - it's not these specific 'transgressions of the law'' that separate people from God for eternity, it is the heart desire to be led by idolatry rather than be led by God that separates people from God for eternity