The ongoing internal conflict in every Christian between the remaining corruption of the "flesh" (Greek sarx — here not mere physical body, but fallen human nature in its hostility to God) and the Holy Spirit who indwells the believer. The classic Pauline treatment is Galatians 5:16-25: "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." The same warfare appears in Romans 7-8 and Romans 6.
The Christian life is a war, not a vacation. Regeneration does not delete the sin nature; it installs a rival King in the believer's heart, and the old resident refuses to leave quietly. Galatians 5:19-21 lists the works of the flesh — sexual immorality, idolatry, sorcery, enmity, strife, jealousy, fits of anger, rivalries, dissensions, divisions, envy, drunkenness, orgies — the predictable output of fallen nature left to itself. Galatians 5:22-23 lists the fruit of the Spirit — love, joy, peace, patience, kindness, goodness, faithfulness, gentleness, self-control — the supernatural output of the Spirit's work in the renewed heart. Both sets of desires coexist in the Christian; the question is always, which is winning today? Paul's strategy for victory: "Walk by the Spirit, and you will not gratify the desires of the flesh" (5:16). Not "fight the flesh directly and maybe you will walk by the Spirit." Walking by the Spirit is the offense; flesh-victory is the byproduct. Fill the house with the Spirit through prayer, Word, fellowship, and obedience; the flesh loses ground where the Spirit advances. "Those who belong to Christ Jesus have crucified the flesh with its passions and desires" (5:24) — positional, accomplished at conversion. Now live consistent with the cross.