Romans 8:9

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.

Indeed, implying the question, "is it really true?" How do you know if INDEED the Spirit of God dwells in you? "To Dwell" in the original language of this passage is oikeō. It means to dwell in or with, not in the sense of a roomate, but in the sense of a husband and wife, where there is a practical effect of the relationship on the lives of the people in the relationship. It is used in 1 Corinthians 7:12,13 like this and then in 1 Timothy 6:16 when Paul is describing the majesty of God as He "dwells in inapproachable light." That's interesting. We are told in Revelation 21:23 that "The city (heaven) had no need of the sun or of the moon to shine in it, for the glory of God illuminated it. The Lamb is its light." And then in Revelation 22:5, "There shall be no night there: They need no lamp nor light of the sun, for the Lord God gives them light." God and the light do not coexist as separate entities. The light is literally part of God and radiating from God who is its source. I'm not saying that we can take the way oikeō is used in 1 Timothy 6:16 and just plop it in to every other place it is used to mean exactly the same thing. As someone who is not independently fluent in Ancient Greek the best I can do is look at the different ways the language is used and gain better understanding of the idea in general by cross-referencing the context of the word in all of Scripture. The implication is that this dwelling is not two separate people living together, but one relationship created by the merging or marriage of two formerly separate things. "If indeed the Spirit of God oikeō you."
Now contrast that idea to the next half of the verse where it states, "Now if anyone does not have the Spirit of Christ, he is not His." Have (echō in the Greek) is a very general term used over 700 times in the New Testament to describe possessions, having different emotions, or being clothed with something. I wonder if there are varying degrees of having the Holy Spirit. I know this is a huge subject and I can only scratch the surface at the moment, but we know we can have the Holy Spirit, be filled with the Holy Spirit, and have the Holy Spirit come upon us and overwhelm us for a specific task. I wonder if this verse can be read to mean, "If the Holy Spirit is merged to who you are so that your entire life is affected, you cannot be in the flesh. But if you don't even have any kind of  ownership by the Holy Sprit, you don't even belong to God." If this is an accurate understanding (dependent on more research on the topic of the Holy Spirit), then that could mean that you can belong to God, but still be in the flesh. In verse 1 it states, "There is therefore now no condemnation to those who are in Christ Jesus, who do not WALK according to the flesh but according to the Spirit." Your walk will show who's in control. If the Spirit is in control, there is no condemnation for you because it is impossible for you to be in the flesh if the Spirit truly permates who you are. Jesus took our condemnation on Himself. Period. I wonder though if there can be times where we choose to walk away from that covering and give our lives back to our flesh. I'm not talking about the loss of salvation, just wondering if we can walk out from the protective and faithfully secure covering of the blood of Jesus.
Why else would Paul say in verse 13, "if by the Spirit you put to death the deeds of the body, you will live," if they didn't need a reminder to continually put the flesh to death?
The more I read it, the less I think that being in the Spirit is being saved and being in the flesh is being unsaved.
The choice seems to be ours. Will we own the Holy Spirit in a way that we can put it on the shelf and walk away when we want to flesh out, or will we be joined to the Holy Spirit so that everywhere we go and in everything we do, the Spirit is part of it?

So what?

How do we think of the Holy Spirit, or God in general for that matter? Do we go to church hoping to get close to God? Do we segregate our lives into different areas like "work" and "family" and "church" where there is little or no mixture between them? Is the Holy Spirit someone we bring with us sometimes in kind of a fuzzy, generalized echō kind of way, or does the Holy Spirit oikeō us? If we received Christ and claimed His righteousness as our own, He has ownership of us. We can still walk in flesh, but after we are saved we become disobedient servants instead of enemies. A disobedient servant is almost worse because they claim to serve their master, but their actions betray him. An enemy can be trusted to be an enemy, but a servant who isn't dedicated to their master is good for nothing and can be trusted for nothing.
If we're going to choose, we need to choose. John 3:16 tells us about the circumstances by which we are able to become servants, but the "other" 3:16 warns us about being disobedient servants once we're in. "So then, because you are lukewarm, and neither cold nor hot, I will vomit you out of My mouth." Revelation 3:16.