Galatians 5:16-20
Gal 5:16 (KJB)
This I say then, Walk in the Spirit, and ye shall not fulfil the lust of the flesh.
Walk - Live or conduct oneself
Paul now tells the Galatians that if they want to stop all the strife in their assembly, they must walk according to the Spirit indwelling them. If they follow the promptings of the Holy Spirit such as do not continue on in the contentions, then one by one there will be peace in the assembly. The Holy Spirit has an ongoing work in the lives of all the believers and if we continue to walk in the flesh, then we will be quenching the Spirit. (1 Th 5:19 KJV) Quench not the Spirit.  The word quench carries with it the idea of “hinder, repress, or extinguish.” If we continue to fight and war with each other, we are repressing the work of the Holy Spirit in our lives and then we will be grieving Him. (Eph 4:30 KJV) And grieve not the holy Spirit of God, whereby ye are sealed unto the day of redemption. The word “grieve” carries with it the meaning of “distress or sorrow.” When Christians fight and try to devour one another, it causes sorrow in the Holy Spirit because He empowers the Christian to live in peace with one another but when we let the flesh rule, we will grieve and quench the Holy Spirit. When we walk in the Spirit, we are not walking in the flesh. The flesh says, return insult for insult. The Spirit walk says do not. The flesh says plot against each other. The Spirit walk says love them. The flesh says get even. The Spirit walk says it is better to suffer loss. How do you know you are walking in the flesh, when you are ruled by the same ill emotions the world exudes such as retaliation, anger, vindictiveness, etc. Walking in the Spirit exudes tolerance, understanding, love, no getting even, etc.
Gal 5:17 (KJB)
For the flesh lusteth against the Spirit, and the Spirit against the flesh: and these are contrary the one to the other: so that ye cannot do the things that ye would.
Lusteth - Longings or desire
Contrary - Opposed or be set against
Here Paul gives the heart of the matter concerning the Spirit and the flesh. The truth is that when a person is unsaved, they have a dead human spirit. When Adam sinned, God pronounced that he would surely die. Now Adam did die physically many years later but God was not speaking about the flesh at that time. Adam died in his spirit, which means that every human being was born with a dead spirit. Now that spirit is dead toward the things of God. When a person becomes saved, it is akin to a person receiving a resurrected soul. Now their soul is not in accord with the flesh. When we were unsaved, our flesh did what it wanted to do because the spirit in us was dead. Now that we have become saved, our spirit is alive unto God but the flesh has not been redeemed. This means that the flesh still desires to sin while the living soul inside us desires to be obedient unto the Lord. This why this verse states that the flesh and the Spirit are in opposition. The Holy Spirit in us is exactly that, Holy, and our flesh is not, therefore they are in opposition to each other, each is subject to a different kingdom. Our flesh still can still serve the kingdom of Satan with all its desires and our resurrected soul serves the Kingdom of God with all its purity. This is why the believer cannot do the things they want to do and that is be totally obedient to God. We try to be obedient and then our flesh gets in the way which causes us to sin. This is why the Christian has such a war in their body and soul. The flesh goes against the Spirit and the Spirit goes against the flesh. And that war will wage until the last day or our home going.  (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.  (Rom 8:8 KJV) So then they that are in the flesh cannot please God.

I am not advocating a form of Gnosticism here.  The Gnostics believed that the flesh was evil and in fact, the heretic Origen, who is touted as a great Church Father even castrated himself.  What I am saying here is that our flesh was not redeemed and therefore it is still subject to sinning.  Whenever we sin we are aligning ourselves with the kingdom of Satan because those who sin are not in accord with God's kingdom and there are only two kingdoms on this earth.  If I were to say that Christians do not sin, I would be advocating Christian Perfection which is not taught in Scripture.  Christ redeemed our souls and made them alive.  Now we who are Christians are able to present our bodies as a living sacrifice in the Kingdom of God.  This is because our souls have been made alive and our physical actions are different now than when we were unsaved.  This does not mean that the flesh is no longer subject to sin and sinful lusts.  You can go out and pass tracts in the afternoon and then go home and sit in front of the TV and drink alcohol or have a cigarette.  On one hand you are presenting your body as a living sacrifice unto God by handing out tracts and then on the other hand you are serving the lusts of the flesh which means you are aligning yourself with the kingdom of Satan.  That is because we are still spiritually inconsistent while in this flesh.  According to Romans 8:8, those in the flesh cannot please God and this is why the unbeliever cannot please God.  Now, when we sin, we are acting like an unbeliever and at that moment, are we pleasing God?  The answer is no because outward sinning is not consistent with the inward transformation which took place at the moment of our salvation.  What I stated has absolutely nothing to do with Gnosticism but totally with the war between the flesh and the Spirit, as taught in the Scriptures alone.

Gal 5:18 (KJB)
But if ye be led of the Spirit, ye are not under the law.
Led - Guided or driven
Those who are saved by grace alone are not under the curse of the law because the sacrifice of Christ was complete, fulfilling the demands of God’s righteous law on behalf of the Elect whom He came to save. Those who are under the law are not being led by the Spirit as the two are in contradistinction to each other.
Gal 5:19 (KJB)
Now the works of the flesh are manifest, which are these; Adultery, fornication, uncleanness, lasciviousness,
Manifest - Visible, clear, or plain
Now Paul gives a list of the sins of the flesh and these are the major sins which makes the work of the flesh obvious in a person’s life.
Adultery - Illicit intercourse between two people, one being married. This also applies to a divorced person too. A person may be divorced in the eyes of the state but in God’s eyes if their spouse still lives and they marry another and have intercourse, it is adultery. (Luke 16:18 KJV) Whosoever putteth away his wife, and marrieth another, committeth adultery: and whosoever marrieth her that is put away from her husband committeth adultery.
Fornication - Sexual unchastely or religious idolatry - This word is used to define those false religions that people cling to, especially the ones that circumvent the cross of Christ and His deity. This would definitely have included the Judaizers in Galatia.
Uncleanness - Ritual impurity or immorality - Jesus also used this term in reference to human commandments. (Mat 23:27 KJV) Woe unto you, scribes and Pharisees, hypocrites! for ye are like unto whited sepulchres, which indeed appear beautiful outward, but are within full of dead men's bones, and of all uncleanness. He used it in reference to the phony religion of the Scribes and Pharisees, which could also be applied to false religions today. Those who are not cleaned by the blood of Christ are unclean no matter how religious they are.
Lasciviousness - Wantonness or licentiousness - This sin is identified with those who have no shame when it comes to committing sin. This sin can be seen in orgies or parades where Sodomite acts are committed right on the street.
Gal 5:20 (KJB)
Idolatry, witchcraft, hatred, variance, emulations, wrath, strife, seditions, heresies,
Idolatry - This is the practice of worshipping idols which would have familiar to the Galatians. It can be applied to today as many religions have made idols. The earth is made an idol by New Agers and those who worship mother earth or “Gaia” worship. Teachers or preachers are made as idols as they are looked upon for final interpretations. An idol can be anything which comes between God and His children.
Witchcraft - Sorcery or magic - We see a rise in witchcraft today under the term “Wiccan.” They divide this into black witchcraft and white witchcraft but both are equally satanic.
Hatred - Enmity or hostility - We see this daily as car bombs go off in market places or people who go into malls and shoot. All one needs to do is drive a car for a while and you will experience someone’s hatred.
Variance - Strife, discord, or contention - As we saw in the book of Galatians that the false teachers were causing much strife in the Galatian church. We must take a lesson from them because false teachings can cause all kinds of consternation. The sorry thing is that in many cases, the Christians who bring the truth, are the ones who are normally escorted out the door.
Emulations - Zealous, jealous, or envious - We see this as many jealous people pervade the churches. In Galatia, the false teachers had a jealousy of the leaders and wanted to depose the leaders who taught grace and replace them with themselves so they can bring the people under the law. The Greek word carries with it the meaning of “being zealous but with a wrong goal.” These false teachers were zealous for the law but did not realize they were adulterating the true way of salvation by trying to mix law with grace. I remember one time I was teaching a new believers class at a church and had about 20 members. The assistant pastor at that time was jealous and took over my class. Needless to say, the class finally dissolved because people were pulling out of it. These people wanted bible teachings not personal drivel.
Wrath - Great anger or rage - Many times when there is great anger in a church it is normally leveled at those who bring the truth and that is a great error.
Strife - Selfish ambition - Here is the cause of much strife in any church. When a person or even a church leader has selfish ambitions to propel themselves to notoriety, they will not care who they step on to get what they want. The death knell of any church is when the pastor attempts to be the kingpin because that attitude trickles down and affects everyone, even the spirituality of the church is ruined.
Seditions - Dissension - Anytime there is dissension in the church it normally escalates to all out war and can lead to the ruin of a congregation. The false teachers were bringing in a different teaching than what was there and this caused serious problems creating factions in the church.
Heresies - Sects or division - Another major flesh evidence is the bringing in of heresy. It was absolute heresy to teach that one must be under the law to gain salvation. Heresy will divide a church today as it did at the beginning. Heresy must be checked so no one will fall prey to it because normally heresy is very close to the truth but close examination would reveal the faults.