9 But you are not in the flesh but in the Spirit, if indeed the Spirit of God dwells in you. Now if anyone does not have the Spirit of Christ, he is not His.
10 And if Christ is in you, the body is dead because of sin, but the Spirit is life because of righteousness.
The Holy Spirit is both the Spirit of God and the Spirit of Christ.
Blessed be the God and Father of our Lord Jesus Christ, who hath blessed us with all spiritual blessings in heavenly places in Christ (Ephesians 1:3).
That He would grant you, according to the riches of His glory, to be strengthened with might by His Spirit in the inner man; that Christ may dwell in your hearts by faith; that ye, being rooted and grounded in love (Ephesians 3:16-17).
Examine yourselves, whether ye be in the faith; prove your own selves. Know ye not your own selves, how that Jesus Christ is in you, except ye be reprobates (2 Corinthians 13:5).
The indwelling of the Spirit is the indwelling of Christ Himself.
We live in dying bodies, but our spirits are secure eternally in life because of the righteousness of Christ.
In contrast, God sees our sinful bodies as already dead.