In Romans 8:9, Paul says that a mark of believers is that they live “in the realm of the Spirit.” Here is a look at the context of this statement:
In Romans 8:1, the apostle Paul declares, “There is therefore now no condemnation for those who are in Christ Jesus.” This verse establishes the central theme of the eighth chapter, namely, the assurance of salvation for those who are in Christ Jesus. Believers are free from the condemnation of sin because they have been united with Christ through faith.
Romans 8:2 explains why there is no condemnation for those who are in Christ Jesus. The law of the Spirit of life has set believers free from the law of sin and death. In other words, the Holy Spirit empowers believers to obey God’s moral law rather than being enslaved to the power of sin and death.
Romans 8:3 explains that the Law of Moses, which was given to Israel, was powerless to save people from sin because of human frailty. However, God sent His own Son to become a human being and offer Himself as a sacrifice for sin (cf. John 10:18). In doing so, God condemned sin in the flesh and freed believers from the power of sin.
Romans 8:4 explains that Jesus offered Himself as a sacrifice for sin to fulfill the requirements of the law—the requirements being perfect obedience to God (cf. Matthew 5:17). Those who have been saved by grace through faith in Christ (cf. Ephesians 2:89) are empowered to walk according to the Spirit and fulfill the requirements of the law through the sanctifying power of the Holy Spirit.
Romans 8:5 contrasts those who live according to the flesh and those who live according to the Spirit. Those who live according to the flesh set their minds on sinful things, but those who live according to the Spirit set their minds on heavenly things.
Romans 8:6 distinguishes between a mind set on the flesh, which leads to death, and a mind set on the Spirit, which leads to life and peace. Believers are urged to set their minds on the things of the Spirit and have life and peace (cf. Colossians 3:1–4).
Romans 8:7 explains that a mind set on the flesh is hostile toward God and cannot obey His law. This is because the flesh is corrupted by sin and is unable to please God (cf. James 4:4).
Romans 8:8 bluntly states that those who are in the flesh cannot please God. In other words, those who live according to their own sinful desires and not according to the Spirit are unable to please God. It is only through faith in Christ and the work of the Holy Spirit that believers can please God (cf. Colossians 1:10 and 1 John 3:22).
Romans 8:9 contrasts the realm of the flesh with the realm of the Spirit. The NIV translation uses the word realm to describe two different spheres: the sphere of the flesh and the sphere of the Spirit. A realm is where a person operates, whether it is the area influenced by the flesh or the area influenced by the Spirit. If a person has the Spirit of God within, then he is living in the realm of the Spirit, which is the domain of the Holy Spirit. But if a person does not have the Spirit of God, then he remains in the realm of the flesh and does not belong to God.
The New Living Translation of Romans 8:9 associates the realm of the Spirit with living under the Spirit’s control: “But you are not controlled by your sinful nature. You are controlled by the Spirit if you have the Spirit of God living in you. (And remember that those who do not have the Spirit of Christ living in them do not belong to him at all.)”
Romans 8 reminds us that salvation is primarily a work of God in us. It is the Holy Spirit who indwells and sanctifies us to be more like Christ. We cannot do it on our own. We must allow ourselves to be controlled by the sway, power, and influence of the Holy Spirit.