"This I say then, Walk in the Spirit, and ye shall not fulfill the lust of the flesh" (Galatians 5:16).
The conflict between flesh and spirit is a frequent theme in Scripture, beginning way back in the antediluvian period: "And the LORD said, My spirit shall not always strive with man, for that he also is flesh" (Genesis 6:3). The "flesh," of course, refers to the physical body with all its feelings and appetites, while man's "spirit" refers especially to his spiritual nature with its ability to understand and communicate in terms of spiritual and moral values, along with its potential ability to have fellowship with God.
Because of sin, however, the natural man is spiritually "dead in trespasses and sins" (Ephesians 2:1), and "they that are in the flesh cannot please God" (Romans 8:8). When the flesh dominates, even the apostle Paul would have to say, "I know that in me (that is, in my flesh,) dwelleth no good thing" (Romans 7:18). This aspect of human nature became so dominant in the antediluvian world that "all flesh had corrupted his way upon the earth" (Genesis 6:12), and God had to wash the world clean with the Flood.
Now, however, the substitutionary death of Christ brings salvation and spiritual life to all who receive Him by the Holy Spirit. "If Christ be in you, the body is dead because of sin; but the Spirit is life because of righteousness. But if the Spirit of Him that raised up Jesus from the dead dwell in you, He that raised up Christ from the dead shall also quicken your mortal bodies by His Spirit that dwelleth in you" (Romans 8:10-11). By the Lord Jesus Christ, the human spirit is made alive right now, through the indwelling Holy Spirit, and the body's resurrection is promised when Christ returns.
"They that are Christ's have crucified the flesh." The daily challenge to the believer is this: "If we live in the Spirit, let us also walk in the Spirit" (Galatians 5:24-25). HMM