Notice four things that Paul tells the Christians at Rome:
1. They live according to the Spirit if they have the Holy Spirit within. (8.9a)
This is especially important to our understanding of Paul; the concept of the willfully sinning Christian, the black sheep in the family of God, was without Paul's sanction. He simply had no category in his mind to describe saints who lived like sinners. It's a given for him that saints act like Christians.
2. If they don't have the Spirit, they aren't Christians to begin with. (8.9b)
Another milestone in his thought involves the Holy Spirit. All Christians have him reigning within them. To be a Christian means that one has the Spirit. ["the Spirit of Christ" is another phrase for the Holy Spirit. (Acts 16.7; Phil 1.19)]
3. They still have to suffer some of the after-effects of Adam's fall from grace but are not without the Spirit's ministry. (8.10,11)
Unless Christ returns on the Day of Resurrection and Judgment while we still are living, we all will die, and without exception. Even though we are not in Adam anymore but are the new creation in Christ, nevertheless, we still have some hang-over effects of Adam's sin. One is physical death.
However, the Spirit will enable us to live before we are given new resurrection bodies; he is our Spiritual strength now. We are not without his power to live in holiness during our days before the Day.
4. They still must be warned to remain in the Holy Spirit lest they apostatize. (8.12,13)
Notice the warning from Paul. The two categories "in the flesh"/"in the Spirit" are not forever fixed. We rejoice that sinners can become saints! However, the converse is true: saints can become sinners. We are to remain in Christ as productive Christians lest spiritual destruction come upon us.
"I am the true vine, and my Father is the vinedresser. Every branch of mine that does not bear fruit he takes away, and every branch that does bear fruit he prunes, that it may bear more fruit. Already you are clean because of the word that I have spoken to you. Abide in me, and I in you. As the branch cannot bear fruit by itself, unless it abides in the vine, neither can you, unless you abide in me. I am the vine; you are the branches. Whoever abides in me and I in him, he it is that bears much fruit, for apart from me you can do nothing. If anyone does not abide in me he is thrown away like a branch and withers; and the branches are gathered, thrown into the fire, and burned." (John 15.1-6 ESV)
Given the alternative I'd rather be in the Spirit.