A.T. Robertson Commentary Romans 8:9

"But ye are not in the flesh but in the Spirit, if so be that the Spirit of God dwelleth in you. But if any man hath not the Spirit of Christ, he is none of his." — Romans 8:9 (ASV)

Not in the flesh (ουκ εν σαρκ). Not sold under sin (7:14) any more.

But in the spirit (αλλα εν πνευματ). Probably, "in the Holy Spirit." It is not Pantheism or Buddhism that Paul here teaches, but the mystical union of the believer with Christ in the Holy Spirit.

If so be that (ειπερ). "If as is the fact" (cf. 3:30).

The Spirit of Christ (πνευμα Χριστου). The same as "the Spirit of God" just before. See also Php 1:19; 1 Peter 1:11. Incidental argument for the Deity of Christ and probably the meaning of 2 Corinthians 3:18 "the Spirit of the Lord." Condition of first class, assumed as true.