The New Testament writers frequently contrast flesh (Greek sarx) and Spirit/spirit (Greek pneuma). But the contrasts are not all the same. It may be helpful to distinguish ten different contrasts (though some of them overlap).
Physical vs. Spiritual Aspect
Paul exhorts, "Let us cleanse ourselves from every defilement of body [sarx] and spirit" (2 Cor. 7:1). His point is that sin contaminates our whole being, which he views here as having two aspects: physical (external) and spiritual (internal). Paul uses this dichotomy elsewhere: "Though I am absent in body [sarx], yet I am with you in spirit" (Col. 2:5; see 1 Cor. 5:3); "you are to deliver this man to Satan for the destruction of the flesh, so that his spirit may be saved in the day of the Lord" (1 Cor. 5:5).
Physical Weakness vs. Noble Desires
"The spirit indeed is willing, but the flesh is weak" (Matt. 26:41; Mark 14:38). We can be physically weak in a way that makes it hard to do what is right while nobly desiring to do what is right.
Physical Body vs. Non-physical Person
"See my hands and my feet, that it is I myself. Touch me, and see. For a spirit does not have flesh and bones as you see that I have" (Luke 24:39). After Jesus rose from the dead, He had to convince His disciples that He had a physical body and was not merely a ghost or non-physical person.
Paul contrasts physical and spiritual warfare: "For we do not wrestle against flesh and blood, but…against the spiritual forces [Greek pneumatikos, an adjectival form of pneuma] of evil in the heavenly places" (Eph. 6:12).
Physical Body vs. Holy Spirit
Christ appeared and was killed in a body, and the Spirit resurrected Him: "He was manifested in the flesh, vindicated by the Spirit" (1 Tim. 3:16). Christ was "put to death in the body [sarx] but made alive in the Spirit" (1 Peter 3:18 NIV; see 4:6).
Perishable vs. Imperishable Body
In 1 Corinthians 15:35-57, Paul contrasts our perishable (physical) body with our future imperishable (physical) resurrection body: "Flesh and blood cannot inherit the kingdom of God, nor does the perishable inherit the imperishable" (v. 50). "It is sown a natural body [Greek sōma, a synonym of sarx]; it is raised a spiritual [pneumatikos] body [sōma]. If there is a natural body [sōma], there is also a spiritual body [pneumatikos]" (v. 44). In this case, the natural body and the spiritual body are both physical; the Bible never calls the body itself evil. The difference between them is that the spiritual, or resurrection, body will never die.
Physical Union vs. Spiritual Union
"Do you not know that he who is joined to a prostitute becomes one body with her? For, as it is written, ‘The two will become one flesh.' But he who is joined to the Lord becomes one spirit with him" (1 Cor. 6:16-17). One reason that Christians must not have sex with prostitutes is that such physical union is incompatible with their spiritual union with Christ because their body belongs to Christ (see 1 Cor. 6:12-20).
Spiritual Death vs. Spiritual Life
"That which is born of the flesh is flesh, and that which is born of the Spirit is spirit" (John 3:6). The principle is that like generates like. Humans reproduce more spiritually dead humans, and the Spirit produces people who are spiritually alive.
Human Inability vs. the Holy Spirit's Ability
1. "That which is born of the flesh is flesh, and that which is born of the Spirit is spirit" (John 3:6).
2. "It is the Spirit who gives life; the flesh is no help at all. The words that I have spoken to you are spirit and life" (John 6:63).
3. "No one is a Jew who is merely one outwardly, nor is circumcision outward and physical. But a Jew is one inwardly, and circumcision is a matter of the heart, by the Spirit, not by the letter" (Rom. 2:28-29a).
Humans cannot produce eternal life. Only God's Spirit can (John 1:13). This underscores what Paul writes in Galatians 4:29: "He who was born according to the flesh [Ishmael] persecuted him who was born according to the Spirit [Isaac]."
The same principle applies to Christian living: "Are you so foolish? Having begun by the Spirit, are you now being perfected by the flesh?" (Gal. 3:3; see also Phil. 3:3-4). The Spirit produces life so that people trust Christ initially at conversion and throughout their lives.
The Realm of the Non-Christian vs. the Realm of the Christian
The contrast here is being "in the flesh" versus "in the Spirit." Only non-Christians live "in the flesh" in this sense (parallel to the "old self" in Rom. 6:6; Eph. 4:22; Col. 3:9), and only Christians live "in the Spirit."
"For while we were living in the flesh, our sinful passions, aroused by the law, were at work in our members to bear fruit for death. But now we are released from the law, having died to that which held us captive, so that we serve in the new way of the Spirit and not in the old way of the written code" (Rom. 7:5-6).
"In order that the righteous requirement of the law might be fulfilled in us, who walk not according to the flesh but according to the Spirit. 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. For to set the mind on the flesh is death, but to set the mind on the Spirit is life and peace. For the mind that is set on the flesh is hostile to God, for it does not submit to God's law; indeed, it cannot. Those who are in the flesh cannot please God. You, however, are not in the flesh but in the Spirit, if in fact the Spirit of God dwells in you. Anyone who does not have the Spirit of Christ does not belong to him…. So then, brothers, we are debtors, not to the flesh, to live according to the flesh. For if you live according to the flesh you will die, but if by the Spirit you put to death the deeds of the body, you will live" (Rom. 8:4-13).
Paul's sowing-and-reaping principle fits here, for "corruption" contrasts with "eternal life": "The one who sows to his own flesh will from the flesh reap corruption, but the one who sows to the Spirit will from the Spirit reap eternal life" (Gal. 6:8).
Sinful Disposition vs. the Holy Spirit
Prior to their glorification, Christians are engaged in a lifelong struggle against "the flesh":
"For you were called to freedom, brothers. Only do not use your freedom as an opportunity for the flesh, but through love serve one another…. But I say, walk by the Spirit, and you will not gratify the desires of the flesh. 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. But if you are led by the Spirit, you are not under the law…. And those who belong to Christ Jesus have crucified the flesh with its passions and desires. If we live by the Spirit, let us also keep in step with the Spirit" (Gal. 5:13, 16-25; see Rom. 7:18; 1 Peter 2:11).
People whose lifestyle is characterized by the flesh will not inherit the kingdom of God (Gal. 5:19-21). Believers, therefore, should "make no provision for the flesh, to gratify its desires" (Rom. 13:14).