"Those who are in the flesh cannot please God. But you are not in the flesh but in the Spirit, if indeed the Spirit of God dwells in you." (8:8-9)
To literally live "in the flesh" means that we are slaves to sin--in bondage to sin. One can only go so far "in the flesh" because one is living in their own power. To live "in the Spirit" means we are under the grace of God, by the sacrifice of His Son, the One who overcame death and sin, and have been given the gift of life and freedom from the power of sin.
So many times I think it is a lack of belief--belief that we who have accepted the sacrifice of Christ are indeed dead to sin. Sin no longer has a right. Sin no longer is our master. We simply need to believe. Just a couple of chapters earlier Paul says, "reckon yourselves to be dead indeed to sin, but alive to God in Christ Jesus our Lord. Therefore do not let sin reign in your mortal body, that you should obey it in its lusts...For sin shall not have dominion over you, for you are not under law but under grace." (Romans 6:11-14)
Reckon it so.
As individuals "in the Spirit" we have a choice--let sin reign or not. Sin now only has the rights and powers we choose to give it. "How?" you ask, "How do I not let it reign?" I think the answer comes back to our thought lives--bringing every thought captive to obedience. This done only in the power of God Himself.
Freedom and victory are gifts from the God of the universe to those who believe. The question is, have we accepted them.
In His grace, press on.
For further nourishment: Galatians 2:11-21