16 But I say, walk by the Spirit, and you will not gratify the desires of the flesh. 17 For 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. 18 But if you are led by the Spirit, you are not under the law. 19 Now the works of the flesh are evident: sexual immorality, impurity, sensuality, 20 idolatry, sorcery, enmity, strife, jealousy, fits of anger, rivalries, dissensions,divisions, 21 envy,[d] drunkenness, orgies, and things like these. I warn you, as I warned you before, that those who do such things will not inherit the kingdom of God. 22 But the fruit of the Spirit is love, joy, peace, patience, kindness, goodness, faithfulness, 23 gentleness, self-control; against such things there is no law. 24 And those who belong to Christ Jesus have crucified the flesh with its passions and desires.
What does it mean to "walk by the Spirit?" I guess in part, that depends on your theology, and how much of a choice you have in the matter. I believe that we do possess certain choices, while God orchestrates the big picture. So walking by the Spirit is a choice that the Christian must make, and the goal is for the Christian to experience a greater joy and closer relationship with God by intentionally seeking Him. And the closer that relationship, the lesser our desire to do those things that would lessen that joy.
The flesh and the Spirit are at war. The Bible makes that clear. And I am flesh, so my desires are at war with the Spirit. As a being of flesh, I am out to please myself. But as a being of flesh, I am also blind to the things of the Spirit, and the joy that can be found in surrender and obedience.
I don't want the word obedience to be misinterpreted. It is not about following an external set of rules. If we are in submission to the Spirit, we are not under the law. But because of our relationship, our love, our joy, we will not do things that go against the Spirit.
The fruits of the flesh are obvious. Or at least they would be if we weren't so blind. So Paul lists them for us in case there was any doubt. The fruit of the Spirit should be obvious too. But again, just in case there is any doubt, Paul lists them. These are not a law, but rather a fulfillment of a relationship with God.
He closes with this statement: "And those who belong to Christ Jesus have crucified the flesh with its passions and desires." That doesn't sound optional. But then I read about the struggle with the flesh in Romans. We are not yet perfect, but we cannot take our walk with the Spirit lightly.
If we are struggling with this, I am reminded of a mechanic's saying that somewhat applies here. "If at first you don't succeed, a bigger hammer is what you need." Excuse me, while I run to the hardware store.