Galatians 5:16-18

So I say, live by the Spirit, and you will not gratify the desires of the sinful nature. For the sinful nature desires what is contrary to the Spirit, and the Spirit what is contrary to the sinful nature. They are in conflict with each other, so that you do not do what you want.

(v16-17, NIV)

Paul has just made a point that we should serve one another in love as opposed to gratifying the sinful nature. Though we are living justified by grace, we should not use this freedom to simply gratify all our sinful desires. Now, Paul makes a similar contrast, but this time it's between the Spirit and the flesh. How do we not gratify the desires of the flesh? The answer is to live by the Spirit. The Spirit and the flesh are opposites. Since they are opposites, you cannot be both living in the Spirit and living in the flesh. You must choose one or the other. As a result, you should live by the Spirit to keep from indulging in the sinful nature.

We know that the law is spiritual; but I am unspiritual, sold as a slave to sin. I do not understand what I do. For what I want to do I do not do, but what I hate I do. And if I do what I do not want to do, I agree that the law is good. As it is, it is no longer I myself who do it, but it is sin living in me. I know that nothing good lives in me, that is, in my sinful nature. For I have the desire to do what is good, but I cannot carry it out. For what I do is not the good I want to do; no, the evil I do not want to do—this I keep on doing. Now if I do what I do not want to do, it is no longer I who do it, but it is sin living in me that does it.

(Romans 7:14-20, NIV)

Because of this conflict between the Spirit and the flesh, we end up doing what we do not want to do. This conflict is similar to the one found in Romans 7. Here, Paul speaks about his desire to do good, but his sinful nature within him is what causes him to sin. When we are indulging in sin, we are living by our flesh. However, if we live by the Spirit, we cease to indulge in sin.

But if you are led by the Spirit, you are not under law.

(v18, NIV)

Those false teachers were teaching that it is by following the law that we are justified. However, Paul firmly opposes this idea and says that it is by grace through faith in Jesus that we are justified. Another contrast Paul makes is the contrast between being led by the Spirit and being under the law. If you follow the teaching of these false teachers, you are living under the law. However, we are to live by the Spirit, which is contrast to living under the law. Also, living by the Spirit is contrast to living by the flesh. When we live by the flesh, we indulge in our sinful nature. As a result, I gather that those false teachers are indulging in their sinful natures since they are living by the law and not by the Spirit.