Romans 8 is a long and winding, yet moving line of reasoning of Paul...he's leading us somewhere...can you guess where? The main character of this part of the letter is the Spirit. Interestingly, at no point in this chapter is the word "Holy" attached with "Spirit", though he does start the next chapter adjuring the Holy Spirit to confirm the truth of a statement he has just made.
Prior to chapter 8, I only found a few references to the Spirit in the other seven chapters, once in chapter 7 (Spirit), once in chapter 5 (Holy Spirit), and once in chapter 1 (Spirit of holiness). Following chapter 8, there are a few more references: once in chapter 9, as laready noted, once in chapter 14 (Holy Spirit), four times in chapter 15 (twice Holy Spirit, once Spirit of God, once Spirit).
Who and what is the Holy Spirit? Paul gives us some illumination, but not too much - if we find any, it is mostly in chapter 8, it is almost as if he concentrated all his comments about the Spirit into one section, and then moved on. According to chapter 8, what does the Spirit do? That might help us figure out who the Spirit is.
vs1 the Spirit gives life (think Genesis account...spirit and breath and wind in Hebrew are all the same word)
vs4-5 we can live according to the Spirit
vs 5 we can desire what the Spirit desires
vs 6 the Spirit can control our mind (the outcome being joy and peace)
vs9 we are in the Spirit if the Spirit of God lives in us (Paul is diving deep into philosophy, metaphysics, theology...)
vs 9 we can have the Spirit of Christ (as a possession? as a body part? as an soul-attachment?)
vs 10 the Spirit gives life...again
vs 11 the Spirit raised Jesus from the dead
vs 11 this Spirit can live in you
vs13 by the Spirit you can put to death the misdeeds of the body
vs 14 you can be led by the Spirit (think Jesus' 40 day desert journey)
vs 15 the Spirit does not make you a slave that lives in fear
vs 15 the Spirit brought about your adoption as a son/daughter of God
vs 16 the Spirit testifies with our spirit
vs 23 the Spirit bears fruit...
vs 26 the Spirit helps us in our weakness
vs 26 the Spirit intercedes for us with wordless groans
vs 27 the Spirit interedes according to God's will
The Trinity is our overarching theme to describe the relationship and identity of God the Father, God the Son, and God the Spirit. The word "trinity" is never used in Scripture to describe the relationship, it is our word to bring the persons together. In chapter eight we have all three referenced numerous times. God seems to be the initiator, the Son and the Spirit are agents/means/doers of God's desires and intent for humankind. The Spirit is present amongst us since Jesus has come and gone and promised to return...and we still wait...we wait with the Spirit within us...we also await for God the Father to dwell amongst us...and we have his Spirit within us as well as we wait..the Spirit of the Father and the Son are the same Spirit.
Based on what Paul says the Spirit can do, we ought to pay more attention to the role of the Spirit in our lives.
Does your pace of life leave you out of breath? Get the Spirit.
Does your life continually have the wind sucked out of its sails? Get the Spirit.
Do you feel enslaved to a pattern of living that results in dying relationships, unhealthy bodies, and restless hearts/minds? Get the Spirit.
How do you get the Spirit of God/Jesus Christ/holiness?
Read through Romans 8 again very care-fully. Then go back and re-read chptrs 3, 5, 6, 7 again. And then ask for the Spirit.
Then read Matthew 5,6,7.
Then read James 1.
Then read Psalm 119:1-8
And enjoy the Spirit!