“. . . in Romans 8:9,10 those who ‘belong to the Spirit’ (v. 9d, i.e., are ‘in Christ’) are at the same time those ‘in the Spirit’ (v. 9a), those in whom ‘the Spirit of God dwells’ (the Spirit in you, v. 9b), and those in whom Christ dwells (‘Christ in you,’ v. 10a), all because of, to emphasize it again, who the Spirit is (the Spirit of Christ–v. 9c), and who Christ is (the life-giving Spirit–I Cor. 15:45).” (Richard B. Gaffin, Jr., Perspectives on Pentecost, p. 21).
“. . . what Paul [means] by the Spirit, is not the mind or understanding. . . but a celestial gift; for he shows that those are spiritual, not such as obey reason through their own will, but such as God rules by his Spirit. (John Calvin, Commentary on Romans at 8:9).