The Apostle Paul invested blood, sweat, and tears into the churches at Galatia. According to Acts 14:19-20, Paul had been stoned to the point of death in his attempt to plant and strengthen the churches in that area. Later he writes Galatians, an important letter that demonstrates how salvation is a gift of God's grace, and is not ever a result of doing good works or following rules. Now in Galatians 5:16, Paul gives this Biblical command to all saved people: Walk in the Spirit. He has invested too much in these people for them to get saved but then live in sin. "Walking in the Spirit" means to keep in consistent step with the Holy Spirit's leadership in our lives. We must recognize that God has a will for us, and a plan for us. If our values do not match God's, then we will be out of step with His leadership. Our relationship with God must define how we make life's decisions.
According to Galatians 5:16, it is impossible to have a healthy relationship with God while pursuing a sinful lifestyle. "Walk in the Spirit and ye shall not fulfill the lust of the flesh." The word, "not" is a double negative in Greek. We aint never supposed to use double negatives in English... it's just bad English. But it is good Greek. Paul does this to show a STRONG point: When you walk in the Spirit, you will never ever fulfill the lust of the flesh.
What is absent in this verse is the reality of temptation. When we walk in the Spirit, we will not ever fulfill the lust of the flesh, though we will still be tempted by our flesh. When we seek to fulfill our fleshly desires, it is a clear indication that we have ceased to walk in the Spirit. As believers, our relationship with God will always remain intact, but any sin we pursue will damage and weaken than relationship. When we choose to daily walk with God, He will keep us strong in the face of temptation every time.