In his various letters, Paul often uses the word flesh, but he does not really define what he means by it. He sees the flesh as an obstacle to the Christian life. Paul writes about the problems of the flesh, so people who have chosen to follow Jesus need to understand what it is.

We should avoid the Greek view that the soul is good and the body is bad. Paul was not going down that path. The reality is that most sin begins with a thought in our minds and happens when we make a choice with our will to do the thing that we are tempted to do. The spiritual powers of evil mostly work on our minds to persuade or deceive us into sinning.

Given that "the flesh" is not the body, we need a clear understanding of what Paul meant when he used that expression. The Greek word is "sarx". A detailed analysis of some New Testament examples shows that Paul uses the word in a variety of different ways, but they are all related.

The various meanings of the expression "the flesh" outlined above are all linked. They are different expressions of the physical aspects of life. Some, like circumcision, are specifically physical. Others are more loosely linked. From these examples, we can see that the flesh is not a segment of the human soul. It is not another word for our physical bodies because although we can sin with our bodies. It is not our physical body that causes us to sin.

Inherited Ethnic Status

Paul increases his use of the word "flesh" in his letter to the Romans. He clarified an important aspect of his meaning at the beginning of the letter, where he writes about Jesus,

who was born of a descendant of David according to the flesh (Rom 1:3).
The expression "according to the flesh (kata sarx) refers to Jesus' family line. He was a descendant of David. Paul says something similar about himself in his letter to the Philippians when he has reasons for confidence in the flesh.
If someone else thinks they have reasons to put confidence in the flesh, I have more: circumcised on the eighth day, of the people of Israel, of the tribe of Benjamin, a Hebrew of Hebrews (Phil 3:3—5).
When he said that he had confidence in the flesh (en sarx), he was referring to his family line and his ethnicity. He was born as a Hebrew in the tribe of Benjamin. His confidence in the flesh was confidence in the family situation into which he had been born.

Paul had believed that he was part of God's chosen people because he was born as an Israelite into the tribe of Benjamin. He had believed that his birth gave him special privileges. The flesh that he was born into was what gave him special privilege. He saw his flesh, his birth into the right family and tribe, as the source of his privilege.

The word flesh is used in this sense in several places in the New Testament.

The flesh refers to physical relatives and ancestors. Paul realised after his conversion that he had misunderstood the value of his ethnic inheritance. It did not gain him as much as he had believed.

Bad Spiritual Inheritance

After his encounter with Jesus, Paul's view of his ethnic inheritance changed. He realised that he had been born into a human family that had been enslaved by the spiritual powers of evil. Although he knew the requirements of the law because he was a Jew, he found he did not have the ability to do the good he wanted to do, because he was born as a slave of sin (Rom 6:7,17).

According to the flesh, Paul was also a descendant of Adam and Eve. When they rejected God and submitted to the tempter, they placed themselves under his authority. They unwittingly enslaved themselves to Sin and Death. The children of slaves are born as slaves. So, Paul was not just a descendant of Abraham with the privilege of being part of the chosen people, in the flesh, he was also a descendant of Adam born in slavery to Sin and Death. Fortunately, he had been born again and was set free from slavery to sin.

"The flesh" describes our situation at birth. Like Paul, we are born into slavery to Sin and Death. Like Paul, we struggle to please God because the spiritual powers of evil who have influence in our lives prevent us from doing what is good.

I am speaking in human terms because of the weakness of your flesh... you were slaves of sin (Rom 6:19—20).
We are slaves to sin because of the weakness in our flesh. Just like Paul, we inherit this weakness from our parents. Our parents were partly enslaved by the spiritual powers of evil, so we were enslaved in the same way when we were born under their authority. This problem with our birth (called the flesh) makes us slaves of sin and death.

John explained,

That which has been born of the flesh is flesh (John 3:6).
He confirms that the life that we are born into through our parents is "the flesh".

Pauls declares that we need to be set free from the power of flesh and death.

But now that you have been set free from sin and have become slaves of God, the benefit you reap leads to holiness, and the result is eternal life (Rom 6:22).
Paul had used the term "flesh" in this sense of being born under the control of the spiritual powers of evil and opposed to the working of the Holy Spirit.

Romans 7,8

In the second half of chapter seven and the first half of chapter eight of his letter to the Romans, Paul pushes his use of "the flesh" in the sense of the spiritual legacy that he inherited through his ethnic legacy a step further. He uses the word sixteen times in twenty—five verses. Paul explains how sin took us into captivity.

I am fleshly, sold into bondage under sin (Rom 7:14).
Our forefathers were trapped in bondage to sin, so we were born into the same captivity. Paul explains that in his mind, he wants to serve God's law, but "in the flesh", he is a "slave to the law of sin" (Rom 7:25). A slave can make some choices, but they have very little freedom. Paul finds that his fleshly status prevents him from doing good (Rom 7:18).

God sent his son "in the flesh" to set us free from Sin and Death (Rom 8:3).

He condemned sin in the flesh (Rom 8:3).
His death on the cross destroyed the power of the spiritual powers of evil over those who have put their trust in him. The power that these spiritual powers gained through our birth into slavery to sin was destroyed by the cross. The presence of the Holy Spirit is proof that they are set free.
You are not in the flesh but in the Spirit, if indeed the Spirit of God dwells in you (Rom 8:9).
Once we are born again by the Holy Spirit, our situation at birth (the flesh) no longer controls us. Paul describes the difference to his Roman readers. Many English versions use the word "mind" several times when translating Romans 8. However, Paul uses the Greek verb "phroneo", which means "to have understanding". It can also mean "to think in a certain way". So referring to the mind is a bit strong. Paul is not saying that sin comes from the mind. I have used the word "inclination/inclined" to better reflect the meaning of phroneo in the table below.
Romans Before After
8:4 Live according to the flesh Live according to the Spirit.
8:5 Those who are in accord with the flesh
are inclined to the things of the flesh.		 Those who are in accord with the Spirit
seek the things of the Spirit
8:6 The inclination of the flesh is death, The inclination of the Spirit is life and peace.
8:7 The inclination of the flesh is hostile to God.  
8:8 Those who are in the flesh are not able to please God.  
8;9 In the flesh In the Spirit.
8:12 Debtors to the flesh,
living according to the flesh.		  
8:13 Living according to the flesh,
about to die		 Putting to death the deeds of the body
by the Spirit, you will live.
8:14   Those led by the Spirit
are children of God
The contrast is stark. Those controlled by the flesh (their spiritual inheritance) struggle to serve the Holy Spirit. They are hostile to God and are not able to please him because the spiritual powers of evil still have access to their lives to manipulate them into sinning. Those who are born again of the spirit are reborn as children of God, not as slaves to sin. They seek the things of the spirit because they are led by the Spirit.

Flesh and Spirit

We live in a world that is both physical and spiritual. We tend to be most aware of the physical world because we can see and touch it with our five senses. We think with our brains, so our intellectual life is an aspect of the physical world. However, the spiritual world is equally important.

When God created Adam, he formed him physically, and then breathed on him (spiritised him) so he would be a "living being" (Gen 2:7). As a "living being", he moved in both the physical and spiritual world. If we think of our ability to live in the physical world as being "in the flesh", then he was living in the flesh and the spirit.

When Adam and Eve rejected God, they lost their ability to function in the spiritual world. They began to live "in their flesh" only, most aware of the physical world around them. Entities from the spiritual world could influence them, but they were often not aware of it. They were not fully aware of God's presence around them and within them, sustaining their lives. They were stuck in the physical world. This is what Paul meant by being "in the flesh".

Flesh refers to our physical bodies, including our brains, which are made of flesh and blood. This is why our intellectual life is "in the flesh". Adam and Eve were living in a world that is both physical and spiritual, but as people "in the flesh," they were not fully aware and responsive to the spiritual world, both good and bad.

We inherit our life from our parents, who were "in the flesh", and going back far enough, they inherited "flesh" but not "Spirit" from Adam and Eve. We are born "in the flesh", with the ability to function in the physical world, but with minimal ability to function in the spiritual world. That is why we are all dominated by Sin and Death (Rom 6,7) and the other spiritual powers of evil that attack us.

Our problem in life comes from the way that we are born. We inherit from Adam and Eve the status of being "in the flesh" and not attentive to the spiritual world. That is why we need to be born again of the Spirit. Just as God breathed on Adam and he became a living being, when we trust in Jesus, the Holy Spirit moves on us to make us true "living beings" who can function in both the physical and spiritual worlds.

When we are born again of the Spirit, we change from being in the flesh only (operating in the spiritual world only) to being "in the flesh and the Spirit". This ability to function in both the physical and spiritual world enables us to live to our full potential as true living beings. Jesus defeated the spiritual powers of evil on the cross, which equips those who walk in both "the flesh and the Spirit" to escape from their grip.

Galatians

Paul uses similar language in Galatians 5, where he writes about the struggle between the flesh and the spirit. He begins by warning that believers who give a higher place to their ethnic status will fail Jesus' command to love one another if there are Jews and Gentiles in their church.

Do not turn your freedom into an opportunity for the flesh, but serve one another through love (Gal 5:13).
Loving one another, no matter who belongs to the body of Jesus, is more important than ethnic heritage, even Jewish heritage. Paul challenged the Galatians to walk in the Spirit, not in the flesh. The life of the Spirit is opposite to the life of the flesh.
Walk by the Spirit, and you will not gratify the desires of the flesh. For the flesh desires what is contrary to the Spirit, and the Spirit what is contrary to the flesh. They are opposing each other, so that you are not to do whatever you want (Gal 5:16,17).
The flesh opposes the Spirit. If we do what is contrary to the Spirit's leading, we are doing what we want, which is the same as gratifying the flesh. Paul gives some examples of the works of the flesh.
The works of the flesh are obvious: adultery, 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 (Gal 5:19-21).
A few of these examples are sexual, but most of them are attitudinal, like jealousy, ambition, envy, factions, etc. The works of the flesh are the natural outworking in people of what they choose, without regard to the Holy Spirit. The antidote to the works of the flesh is the fruit of the Spirit (Gal 5:22-23).

Paul concludes his discussion about this topic by declaring that the flesh is dead (our situation at birth no longer applies).

Those who belong to Christ Jesus have crucified the flesh with its passions and desires (Gal 5:24).
If we have died with Jesus, we have put our flesh to death. The state in which we were born has no hold over us. Our old spiritual legacy is dead, so it has lost its hold over us. If we have died and risen with Jesus, the power of the spiritual powers of evil over lives is broken. We are no longer in their captivity, so they cannot manipulate and control us. Our spiritual legacy of slavery to sin has been cancelled. We are children of God, inheriting the life that he has chosen for us.

Conclusion

The word flesh (sarx) is used in a variety of different ways in the New Testament.

Many of these uses are different expressions of the physical aspects of life.

Paul used the word flesh (en sarx) frequently when describing his family line and his ethnicity. He had believed that his birth gave him special privileges. After his revelation of Jesus, he realised that his ethnic inheritance (flesh) was a huge problem, because it empowered the spiritual powers of evil to intervene in his life. Paul had struggled to please God because the spiritual powers of evil had influence in his life through his spiritual inheritance to prevent him from doing what is good.

Some modern translators use the expression "sinful nature" to translate the word "flesh", eg New Living Translation. However, this is misleading because it reads meaning into the Greek word "sarx" that is not are not there. Translating this way makes it seem that humans have a sinful nature that dominates their behaviour. It implies that we have a corrupted nature that prevents us from doing good. This is not true.

The reality is that humans were made in the image of God. We are not born with a corrupt nature. The New Testament uses the word flesh to describe our spiritual inheritance from our parents. We are born under the authority of the spiritual powers of evil, so they attack us from an early age to pervert our lives. The struggles of our early lives leave our souls wounded and weak, meaning that we often choose to do the wrong thing. We have been beaten around spiritually, but the scriptures do not teach that we are born with a sinful nature that controls us.

Just like Paul, we inherit this weakness from our parents. Our parents were partly enslaved by the spiritual powers of evil, so we were enslaved in the same way when we were born under their authority. This problem with our birth (called the flesh) makes us slaves of Sin and Death. Paul had used the term "flesh" in this sense of being born under the control of the spiritual powers of evil and opposed to the working of the Holy Spirit. We need to be set free from their power by being born again of the Spirit.

Jesus' death on the cross destroyed the power of the spiritual powers of evil over those who have put their trust in him. The power that these spiritual powers gained through our birth into slavery to sin was destroyed by the cross. Once we are born again by the Holy Spirit, our situation at birth (the flesh) no longer controls us.