9. But ye are not in the flesh, but in the spirit, if so be that the Spirit of God dwell in you--This does not mean, "if the disposition or mind of God dwell in you"; but "if the Holy Ghost dwell in you" (see 1Co 6:11, 19; 3:16, &c.). (It thus appears that to be "in the spirit" means here to be under the dominion of our own renewed mind; because the indwelling of God's Spirit is given as the evidence that we are "in the spirit").
      Now--"But."
      if any man have not the Spirit of Christ--Again, this does not mean "the disposition or mind of Christ," but the Holy Ghost; here called "the Spirit of Christ," just as He is called "the Spirit of life in Christ Jesus" (see on Ro 8:2). It is as "the Spirit of Christ" that the Holy Ghost takes possession of believers, introducing into them all the gracious, dove-like disposition which dwelt in Him (Mt 3:16; Joh 3:34). Now if any man's heart be void, not of such dispositions, but of the blessed Author of them, "the Spirit of Christ."
      he is none of his--even though intellectually convinced of the truth of Christianity, and in a general sense influence by its spirit. Sharp, solemn statement this!

JFB.