All throughout the book of Galatians, Paul has spoken of freedom and slavery. And even at the start of chapter 5, it starts off with Paul writing "It is for freedom that Christ has set us free. Stand firm, then, and do not let yourselves be burdened again by a yoke of slavery." And this is what Paul has to say... those who are saved by grace through faith in Christ Jesus have freedom. And those who are not saved by grace through faith in Christ Jesus are held in bondage and are slaves!
And so what does this freedom in Christ mean? Does that mean we are free to do whatever we please? We can now go and do whatever we want to do, because we have been set free? By no means! Have a look at what Paul has to say here in his letter. "You, my brothers and sisters, were called to be free. But do not use your freedom to indulge the flesh..." He then goes on to say "So I say, walk by the Spirit, and you will not gratify the desires of the flesh."
When we live in the flesh, we desire to gratify the flesh. However, when we are set free in Christ Jesus, we are set apart, and we should desire the things that He desires! Here Paul says that to desire the things that the Father desires, then we need to be people who walk by the Spirit. Walking by the Spirit suggests to me that we are being active and that we are progressing in the things of God!
And so how can we tell if we are walking in the Spirit? Well those who are people that walk in the Spirit look a lot like Jesus. Jesus himself said that the Holy Spirit would come and speak of Him! Have a look at John 14:16-17, 14:26, 15:26, 16:13-15. When we are people that walk in the Spirit, then we listen to what it is the Holy Spirit is saying as He guides us on the path and in the nature of Jesus!
Paul goes on to say that when we walk by the Spirit, we "will not gratify the desires of the flesh." There is no way that we can fulfil the desires of the flesh as we walk in the Spirit. The two just don't go together! The Holy Spirit doesn't move in us to gratify the flesh, but to teach us about Jesus. It is the key to righteous living! Walk by the Spirit!