2 Corinthians 10:3-4 (NASB)
For though we walk in the flesh, we do not war according to the flesh, for the weapons of our warfare are not of the flesh, but divinely powerful for the destruction of fortresses.
This is a great passage of Scripture. It deals with the believer’s spiritual warfare, a passage that every believer should memorize and chew upon often. It prompts the believer to discipline and gain control over his mind and thoughts. The war is not waged against flesh and blood, against physical and material beings, but against spiritual forces. Spiritual forces lie behind man’s evil and lustful behavior, unbelief and arguments against God. These spiritual forces attack and prey upon the imaginations and thoughts of men. They inject selfish and devilish ideas against God and against other men. And from the energy of these ideas come the evil acts of men. Paul immediately concedes that he “walks in the flesh.” In other words, he is subject to weakness, distress, and suffering. This has been his theme in this letter, and also in his self-description in First Corinthians. Yet he does not concede that he “walks according to the flesh.”
In Ephesians 6:12 (NASB), this same spiritual warfare is described: “For our struggle is not against flesh and blood, but against the rulers, against the powers, against the world forces of this darkness, against the spiritual forces of wickedness in the heavenly places.” Selfish, evil, and lustful ideas are by nature not of God. God is not evil, selfish, or lustful; therefore, such evil thoughts and assessments are often injected into the mind of individuals by spiritual forces that stand opposed to God. 
Paul and other believers “walk in the flesh”; that is, we live in a body just like all people. But we do not war in the flesh. Believers do not fight the struggles of life using only their own strength. Why? Because believers know that there is a spiritual warfare going on for the minds and lives of people. Believers know that the evil forces of the spiritual world lie behind sin and evil; death and hell.
These verses that were chosen for use to memorize are “Fighter verses”. Knowing the Word of God is the tool the Spirit of God uses to impact lives. It is the weapon for spiritual battle. Jesus used the Word of God when He was tempted by Satan in the wilderness. That is how we should use these “fighter verses.”
The believer fights the spiritual battles of this life with spiritual weapons, weapons that are of God and are made mighty and powerful by God. Note that the believer uses spiritual weapons to “destroy fortresses.” The evils of this world are deeply entrenched and fortified. This is clearly seen in the daily newscasts and behavior of people day by day. Evil and its inevitable consequence of destruction and death can never be permanently overcome by carnal weapons. Only the spiritual weapons of God can permanently conquer evil.
The mind is the leader or forerunner of all actions. Romans 8:5 makes it clear: For those who are according to the flesh and are controlled by its unholy desires set their minds on and pursue those things which gratify the flesh, but those who are according to the Spirit and are controlled by the desires of the Spirit set their minds on and seek those things which gratify the [Holy] Spirit.
Our actions are a direct result of our thoughts. If we have a negative mind, we will have a negative life. If, on the other hand, we renew our mind according to God’s Word, we will, as Romans 12:2 promises, prove out in our experience “the good and acceptable and perfect will of God” for our lives.
And A.W. Tozer said that “Spiritual people are indifferent to their feelings—they live by faith in God with little care about their own emotions. They think God’s thoughts and see things as God sees them. They rejoice in Christ and have no confidence in themselves. They are more concerned with obedience than with happiness. This is less romantic, perhaps, but it will stand the test of fire.”
The more you can feed your mind the Word of God the better able are you to withstand the spiritual warfare and the greater your peace will be throughout your life. Spiritual warfare begins with holiness as you examine the three arenas of spiritual warfare that every maturing Christian will face: the mind, the church, and the invisible spiritual realm.