you now exist not in the flesh rather in spirit if indeed the Spirit of God dwells in you, now the one not having the Spirit of Christ doesn't exist of him (Romans 8:9)
Being joined, united, in communion with Christ is a theme throughput the new testament. It has deep meaning and even mystery. We begin with being joined with Christ spiritually. The new covenant, a new will established by the death of Jesus, thus it is called the covenant in his blood, introduces a new way to enter the kingdom of God, by new birth of the Spirit. It also introduces the promise, established on a better oath by the sacrifice of Jesus who gave himself up on behalf of the church, that atones for and appeases God's wrath toward sin and introduces the promised Holy Spirit. Therefore, if you have been born a new, the Spirit of God exists in you and just as it comes in Christ's works, it is the Spirit of Christ and because they exist three as one. So in the new covenant which brings redemption and a new birth we are joined spiritually with Christ.
and some of you existed these [fornicators, idolators, adulterers, homosexuals, thieves, swindlers, greedy, drunkards...] but you were washed, sanctified, justified in the name of the Lord Jesus Christ and in the Spirit of our God (1 Corinthians 6:9-11)
When we were dead in our trespasses and sin God was rich in mercy through his great love and made us alive, raising us and seating us in the heavenlies (Ephesians 2:2-5). Now, we were dead spiritually walking around in a fleshly body and having a soul, but God made us alive, gave us new birth, sealed, adopted us of the Spirit, when we heard the gospel (Ephesians 1:14-5; Romans 8:15-16; Ephesians 1:13-14).
now, if Christ is in you, though the body is dead through sin, the Spirit is life through righteousness (Romans 8:10-11)
We still live in a body in which sin dwells, we await our salvation, the redemption of our bodies (Romans 7:16-24; Romans 8:19-24). The body is still under the law of sin and death, thus dying, but we are free from condemnation by the Spirit of life that dwells within us (Romans 8:1-4). We have a testimony of the Spirit, and blood that we exist redeemed and adopted children of God. There is a third that testifies of water, we will go into this more as we continue our discussion on being joined with Christ into his body and death.
for it says the two exist one in flesh, now, the one joined of the Lord exists one spirit (1 Corinthians 6 17)
As children of the promise (and not all those who are descendants of Abraham are children of the promise, Romans 9:7-8) we have received the promised Holy Spirit. We are joined with Christ spiritually. This as children we know God and that are sins are forgiven, having testimony of Spirit and Blood. Now, as we mature becoming sons we are led by the Spirit to put to death the deeds of the flesh. So along with the Spirit and truth, the truth of baptism were our unity with Christ in his death makes useless the flesh, so we are alive to God becomes a reality also in our bodies as we are joined in his suffering and death. We are joined with Christ in spirit but in this same scripture and that in Ephesians 5, there is a great mystery of how we become joined with Christ in body...