Christians, according to Romans 8:9-11, are not in the flesh. They are in the Spirit. That, Paul would argue, is the difference between those in Christ and those not in Christ. What does having the Spirit mean?
It means “life.” It means transformation now and transformation then. Paul’s issues are always death and life, not hell and heaven (for that narrows the interest of Paul far, far too much). It is all about death and life — life now, life then; death now, death then.
As Wright makes clear, Paul regularly “confuses” being “in the Spirit” and the Spirit being “in us.” And Paul can mix all this up by using “Christ” as he does “Spirit.” I read Wright as saying to those who are bothered by this sort of language as “get over it; it’s the way Paul carves out his theology” (this is not a quotation but a summary of Wright). To be “in Christ” means to dwell in the life created by Jesus’ death and resurrection. The Spirit makes this a power at work in us.
How does one know if he or she has the Spirit? My view is that Paul would say “Let me watch you live. That’ll be the indicator.”
Because believers have sinned, the body they have will die; but grace and the Spirit triumph over that for life.
Wright translates “because of righteousness” at the end of v. 10, which is surprising in its context, as “God’s covenant faithfulness.” One might say it indicates that life results in spite of sin and death because God has taken care of business in the Messiah (and Spirit).