For those who live according to the flesh set their minds on the things of the flesh, but those who live according to the Spirit set their minds on the things of the Spirit. To set the mind on the flesh is death, but to set the mind on the Spirit is life and peace.
What do we fill our minds with will affect our outlook on life, our very life itself. And the choices and difference here is flesh and Spirit. When we gravitate toward the things of the flesh, we will not only be influenced by that, but the flesh can take over. By flesh here, what’s really meant is all that’s not of the Spirit. The world system along with the principalities and powers which are part of that are in that mix. The Spirit involves all that is of God revealed in Christ.
What is emphasized here is perhaps both practice and disposition. We set our minds on what is of God and become acclimated to that. Or else we let ourselves drift into the thoughts and ways of the flesh, that which is in opposition to God.
If we have the Spirit through Christ, then we can set our minds on the things of the Spirit. And actually when you consider this passage (click above link), it is really something of a description of those who are in Christ and thus in the Spirit and live with a mindset given by the Spirit, and those who are in the flesh without the Spirit and therefore live with a mindset of the flesh (see NET footnotes).
A number of scholars believe that this chapter is getting at what the rest of the book addresses as a problem within the church made up of house churches in Rome. They weren’t always getting along, dividing over disputable issues in which Christians can differ. So even though this passage in Romans 8 seems to draw a stark line, it’s not like we as followers of Christ can’t falter and live apart from the Spirit. That is as plain as day in the letter of 1 Corinthians, but plain enough here too, I think.
The Spirit makes the needed difference. But it seems clear enough to me that this is not automatic, but something we’re to practice, to both set our minds on the things of the Spirit as well as on the Spirit. Regardless of what our experience is, we keep on doing that. In and through Jesus.