But you are not in the flesh but in the Spirit, if indeed the Spirit of God dwells in you. Now if anyone does not have the Spirit of Christ, he is not His. (Romans 8:9)
So after a bit of a break from Romans, we are diving back in!
You might want to go back and read the last post on Romans 8:6-8 about the person who cannot please God. Paul continues to build on what he wrote by inspiration of the Holy Spirit.
Christians are not “in the flesh” but are “in the Spirit.”
Paul’s point is not that Christians no longer live in a physical body. He connects to his point in Romans 8:6-8 that Christians are no longer “fleshly minded.” Their mind is to be set on the things of the Spirit and not the flesh (Romans 8:5).
Why?
Because “they have been crucified with Christ” (Galatians 2:20). They were buried with Christ in baptism and raised up to walk in “newness of life” (Romans 6:3-7). The blood of Jesus Christ had done away with their “old man” of sin.
Because of what Jesus has done, the Christian’s life is new. He provides a new way of living and is our new focus.
If then you were raised with Christ, seek those things which are above, where Christ is, sitting at the right hand of God. Set your mind on things above, not on things on the earth. For you died, and your life is hidden with Christ in God. When Christ who is our life appears, then you also will appear with Him in glory. (Colossians 3:1-4)
God lives in the Christian.
After making the point that Christians “are not in the flesh but in the Spirit,” he gave a condition: “If indeed the Spirit of God dwells in you.” Paul’s argument is based upon this condition being true.
Paul expected his audience to consider whether or not the Spirit dwelled in them and…he wanted them to draw the same conclusions he did. Wallace said their response to the condition would probably be something like: “If the Spirit of God dwells in us? Of course He does!” - Pollard, Truth for Today Commentary, 268-269
The fact that the Holy Spirit lives in the Christian is taught in the New Testament (Acts 5:32; 1 Corinthians 3:16-17; 6:18-20).
The word “dwells” (oikeo) means to reside in a place, to live or dwell somewhere. Just as the Christian lives “in the world” and as the soul dwells “in the body,” so also does the Spirit of God live in a person. Paul did not explain how the Spirit of God dwells in us but only that He does. Anything beyond that is pure speculation. - Pollard, Truth for Today Commentary, 269
The Christian belongs to God.
No connection can be had with Jesus Christ unless someone has “the Spirit of Christ.” Those who have “the Spirit of Christ” are “His.”
Christians belong to God - they are His purchased possession. We were purchased with the priceless blood of Jesus Christ - the Lamb of God who died in our place.
For He made Him who knew no sin to be sin for us, that we might become the righteousness of God in Him. (2 Corinthians 5:21)
…knowing that you were not redeemed with corruptible things, like silver or gold, from your aimless conduct received by tradition from your fathers, but with the precious blood of Christ, as of a lamb without blemish and without spot. (1 Peter 1:18-19)
When Christians are buried with Christ in baptism - putting that old man of sin to death and being set free from sin by the blood of Jesus (Romans 6:3-7) - the Holy Spirit seals, or marks us, as now belonging to God.
In Him you also trusted, after you heard the word of truth, the gospel of your salvation; in whom also, having believed, you were sealed with the Holy Spirit of promise, who is the guarantee of our inheritance until the redemption of the purchased possession, to the praise of His glory. (Ephesians 1:13-14)
And since we belong to God now - how then should we live? We should not live in a way that “grieves the Holy Spirit of God” - living like we don’t belong to God (Ephesians 4:30).
God bought us at a price - the blood of His Son. Therefore, we must live our lives in a way that glorifies Him (1 Corinthians 6:20).
Now remember, Paul has been dealing with the accusation that the gospel he proclaims results in more sin - it encourages people to “continue in sin that grace may abound” (Romans 6:1). Paul continues to demonstrate that the gospel of Christ does not encourage people to continue living in sin - just the opposite!
The gospel of Christ teaches us what He has done for us and calls us to live for Him by following His example.
The gospel calls Christians to live a life that glorifies God.
Have a look at Gal 2:20 in KJV. It is the only translation in its proper present tense, not past. It also tells us that we have HIS faith imputed to us, not “by faith in” but “with the faith of”.
I don't think it affects what you are saying, but the phrase "if indeed the Spirit of God dwells in you" can also be translated as "since indeed the Spirit of God dwells in you", reassuring rather than conditional.
This is not something I discovered myself, but rather Richard Longenecker makes a case for it in his Romans commentary. What is otherwise sometimes regarded as "jumbled" Greek might be clarified using this reading:
“You, however, are not controlled by the sinful nature but by the Spirit, since the Spirit of God lives in you. [If anyone does not have the Spirit of Christ, that person does not belong to Christ. And if Christ is in you, your body is indeed dead because of sin, but your spirit is alive because of righteousness.] Since then the Spirit of ‘the One who raised Jesus from the dead’ is living in you, ‘he who raised Christ from the dead’ will also give life to your mortal bodies through his Spirit, who lives in you.”
The statements that follow ("If anyone does not have...") begin with εἰ ("if") rather than εἴπερ ("since" or "if"), and can be understood as parenthetical as indicated by the brackets.
Every translation I checked (six or so) translates the phrase as starting with "if" rather than "since", but I notice that even in English, the word "if" can take on the meaning "provided that".