After discussing walking in the flesh, Paul next turned to the walking in the Spirit.
Romans 8:9-11 (KJV)
But ye are not in the flesh, but in the Spirit, if so be that the Spirit of God dwell in you. Now if any man have not the Spirit of Christ, he is none of his.
And if Christ be in you, the body is dead because of sin; but the Spirit is life because of righteousness.
But if the Spirit of him that raised up Jesus from the dead dwell in you, he that raised up Christ from the dead shall also quicken your mortal bodies by his Spirit that dwelleth in you.
Paul reminded his believing readers that they were not held in bondage to the flesh any longer, but were in the Spirit because the Holy Spirit dwelt within them (verse 9). If the Holy Spirit was not dwelling within, that person was not God's. So often today in churches there is talk about a universal gospel, where, basically, everyone is saved (except maybe for the worst sinners, i.e. Hitler). But this concept is unknown in the Scriptures. Clearly, unless a person is born again (repented and believed), he or she is not God's, but is lost in sin.
And if Yeshua (through the Holy Spirit) is within, the body is dead because of sin (verse 10). Paul was reminding his readers that because of sin, the body must die (the wages of sin are death).When someone becomes a believer, he or she is joined with Yeshua in His death, and in a certain sense the believer's body is then dead, too. The good news is that then the Holy Spirit can dwell within, giving life because of Yeshua's righteousness.
And because the Holy Spirit dwells within the believer, the Holy Spirit will quicken (or make alive) his or her mortal body, just like Yeshua was raised from the dead (verse 11). The body is enabled to live in righteousness and walk in the Spirit.