Summary: Galatians 5:16–18 introduces the passage by setting forth the nature of Spiritual Warfare between the flesh and the Spirit. In this we see 1) The Warfare Command (Galatians 5:16), 2) The Warfare Conflict (Galatians 5:17), and 3) The Warfare Caution (Galatians 5:18).
One of the most tragic elements of warfare is the event of collateral damage. Often in military operations, civilians are caught in the crossfire. Whether it is in Palestine or Iraq, civilians often get hurt between two fighting forces. Sometimes, one side, usually the weaker one, deliberately puts others in harms way in an effort to defend itself. It is truly tragic when someone is in the midst of a battle and does not realize it. They take no action to avoid the conflict and are unaware of the danger.
Every one of us in smack dab in the middle of a battlefield. The weaker force of our flesh wages war against the Spirit. Although a defeated foe, the flesh will attempt to do as much damage to us as possible against the stronger force of the Spirit. In order not to be caught in the crossfire of the battle, we must realize that a battle rages and take appropriate measure. More simply, we may say that ‘the flesh’ stands for what we are by natural birth, ‘the Spirit’ what we become by new birth, the birth of the Spirit. And these two, the flesh and the Spirit, are in sharp opposition to each other. (Stott, J. R. W. (1986). The message of Galatians: Only one way (p. 146). Leicester, England; Downer’s Grove, IL: InterVarsity Press.)
Having just shuddered to contemplate the possible ruin and annihilation of the Galatian churches through their unruly attacks on one another, Paul now moves on to describe the divinely appointed remedy for this grave dilemma (George, Timothy: Galatians. electronic ed. Nashville : Broadman & Holman Publishers, 2001, c1994 (Logos Library System; The New American Commentary 30), S. 385) The remedy is rooted in understanding the problem. Just as Isaac and Ishmael were unable to get along, so the Spirit and the flesh are at war with each other. The battle will be lost if people seek an external set of laws to stop the conflict. The Law was designed to show the problem, not provide the solution. On the other end, the battle will also be lost if people seek to throw off all restraint and be a law unto themselves, to live however they feel at the moment, according to fleshly desire. The solution to the problem is to be ruled internally by the Holy Spirit. There can be no spiritual life unless the Spirit has first worked that life in the believer (1 Corinthians 12:3). And, on the other hand, where the Spirit is active in a person’s life, there will inevitably be spiritual life and a new spiritual nature (John 3:6). (Panning, A. J. (1997). Galatians, Ephesians (p. 97). Milwaukee, WI: Northwestern Pub. House.)
Do you struggle with focus? Do you find yourself being tempted and feel trapped? Are you dissatisfied in your holiness or find it difficult in knowing how to please God? God has provided a means of following him. Believers have the Holy Spirit of God inside them to convict of sin, guide on the path of righteousness, and empower them to do so.
In its most profound yet simple definition, the faithful Christian life is a life lived under the direction and by the power of the Spirit. That is the theme of Galatians 5:16–26. The opening paragraph here in Galatians 5:16–18 introduces the passage by setting forth the nature of Spiritual Warfare between the flesh and the Spirit. In this we see 1) The Warfare Command (Galatians 5:16), 2) The Warfare Conflict (Galatians 5:17), and 3) The Warfare Caution (Galatians 5:18).
Having the Spirit control the Flesh in Spiritual Warfare means understanding:
1) The Warfare Command (Galatians 5:16)
Galatians 5:16  But I say, walk by the Spirit, and you will not gratify the desires of the flesh. (ESV)
Before we can understand the solution, we have to first understand the problem. What is Paul referring to in this text? Paul begins this text with a contrast “But/so”. This word ties in with verses 13–15. The strategy for removing the divisiveness that marred the Galatian church was to serve one another in love, but that too was humanly impossible. People cannot, in their own power, show love to all people at all times. But God has provided the means to meet his commands—the Holy Spirit.( Barton, B. B. (1994). Galatians (p. 177). Wheaton, IL: Tyndale House.)
Believers are called to “walk by the Spirit” (pneumati peripateite). Paul uses the phrase “walk” thirty times in his other epistles as a favorite way to describe the Christian life. It shows action, direction, a goal, and in this verse a companion Guide along the way. Paul had earlier reminded the Galatians of how they received the Holy Spirit upon hearing him preach the message of Christ and his cross (Gal. 3:1–3). Here he was exhorting them to continue the walk they had begun on that occasion. If they continued to walk in the Spirit, they would not be halted by the fleshly appeals of the Judaizers, their own libertine tendencies, or the debilitating disputes within their churches. Although this is the only place in Galatians where the word “walk” is used in this sense, it is a common Pauline designation for one’s daily conduct or lifestyle. In its wider usage the Greek word means not only “to walk” in a general sense but “to walk around after someone or to walk in a particular direction.” (Boles, Kenneth L.: Galatians & Ephesians. Joplin, Mo. : College Press, 1993 (The College Press NIV Commentary), S. Ga 5:16).