While memorizing chapter eight of the book of Romans, verse nine attracted me.
Paul refers to the Holy Spirit in this one verse three times but by first calling Him the Spirit then the Spirit of God and finally the Spirit of Christ. Now, thus far in this chapter Paul has only referred to the Holy Spirit as the Spirit, and Paul mainly refers to Him as the Spirit for the rest of the chapter (eight out of 10 more times). So, this verse seems to be the culminating verse for clarification as to who is this "Spirit." Maybe Paul is clarifying the third person of the Trinity. Maybe he is making it easy on himself by going ahead and clarifying exactly Who he is talking about early on the chapter, so that we all know that when he refers to simply the Spirit for the rest of the chapter he is speaking of Christ's Spirit. He might be trying to inform his audience that his references to the Spirit are not to your own individual spirit or some life force (indeed, most people believe in spirits and the spiritual realm). Instead, Paul specifically narrows it down by letting us know that when he writes of the Spirit, he is referring to the Spirit of God who is also, more specifically, the Spirit of Christ Himself! Paul seems to be slipping in a little, subtle hint about the unity of the Trinity.
You, however, are not in the realm of the flesh but are in the realm of the Spirit, if indeed the Spirit of God lives in you. And if anyone does not have the Spirit of Christ, they do not belong to Christ.
Anyway, just a little observation. I can feel the mystery here, and I want to discover...