What does Romans 8:9 mean?

Paul has been describing the difference between those who live by their own self-reliant, selfish, sinful "flesh" and those who live by the Spirit of God. Paul's description leaves no room for anyone to both live by the flesh and live by Spirit. Christians live by the Spirit, even if we are sometimes distracted by sin. A true believer in Christ can sin (1 John 1:9–10), but sin is not the normal pattern of behavior for someone who is in Christ (1 John 3:4–6). Non-Christians live by the flesh, serving themselves.

Now Paul makes it clear to his readers, Christians living in Rome, that he understands them to be in the Spirit and not in the flesh. Paul identifies them as Christians, with a stipulation: this is true "if" the Spirit of God lives in you. In modern English, we tend to assume that the word "if" implies doubt, when sometimes it simply connects two ideas. This phrase might be better read as a condition which is assumed to be true. In other words, "You are in the Spirit since the Spirit of God dwells in you."

The reverse is also true: If someone does not have the Spirit of Christ, he or she does not belong to Christ. Paul leaves no room for Christians who do not have God's Spirit with them. God gives His Holy Spirit to every Christian. Without the Spirit, we are not Christians (1 Corinthians 3:16; 2 Timothy 1:14).

Notice that this verse very much supports the idea of the Trinity. God is three persons in one: Father, Son, and Holy Spirit. The Spirit here is referred to both as the Spirit of God and the Spirit of Christ, though it is the same being. In addition, the Spirit of God is said to live in Christians in this verse and Christ is said to be "in you" in the following verse. This corresponds to the idea of three different persons in one Godhead.