The Holy Spirit is “Christ in you, the hope of glory,” Paul wrote in Colossians 1:27. It is, according to Abraham Kuyper, a “mystical union with Immanuel.” Our sanctification is achieved by God through our union with Christ. “He who calls you is faithful; he will surely do it” (1 Thess 5:23–24).
The great existence-altering event that happens in our salvation is our union with Christ through His Spirit.
Paul writes in Galatians 2:20: “I have been crucified with Christ. It is no longer I who live, but Christ who lives in me. And the life I now live in the flesh I live by faith in the Son of God, who loved me and gave himself for me.”
To the Romans, he writes:
 Do you not know that all of us who have been baptized into Christ Jesus were baptized into his death?  We were buried therefore with him by baptism into death, in order that, just as Christ was raised from the dead by the glory of the Father, we too might walk in newness of life.  For if we have been united with him in a death like his, we shall certainly be united with him in a resurrection like his (Rom 6:3–5).
Union with Christ through His Spirit is the only way to combat sin in the flesh – to completely defeat it in the eternal and to battle it in the temporal. Sinclair Ferguson writes:
[T]hose who have been baptized into Christ are united to him in such a way that they share in his death, burial, resurrection, ascension and ultimate glorification. This new identity in union with Christ is the groundwork that the Spirit lays for adequately dealing with the continuing presence of sin. On the basis of it, believers are to put off the characteristics of the old man and but on the characteristics of the new, since they have already put on the new man who is being renewed in knowledge in the image of the Creator.”
That union will result in our glorification on the last day.
However, we’re still stuck in the tension of the now and not yet.
We have been given the Holy Spirit and we are now positionally sanctified and set apart by God as His chosen redeemed. Yet the holiness we are promised often can be elusive in the Christian walk.
What is Paul’s answer to this? Paul encourages us in two things: first, to remember the gospel of Jesus Christ and second, to rely on the Holy Spirit – to “walk in a manner worthy of the calling to which you have been called” (Eph 4:1). Paul roots his exhortations to believers in the indicative, our position in Christ. Paul’s imperative, which can flow only from that indicative, is to walk as those called by Christ.
That indicative and imperative are treated as inseparable by Paul: because of Christ,this is who you now are and this is how you walk as one called. Paul uses exhortation, encouragement, and parenesis – Paul implores us to be who we now are because of the Spirit of Christ who dwells in us.
Next: Completed by the Spirit Part 17: The Gospel Brings About All Aspects of Our Salvation
 Abraham Kuyper, The Work of the Holy Spirit (London: Funk and Wagnall’s, 1900), 333.
 Sinclair B. Ferguson, The Holy Spirit (Downers Grove, IL: 1996), 152.