Jamieson, Fausset and Brown on Ephesians 3:6

6. Translate, "That the Gentiles are," &c. "and fellow members of the same body, and fellow partakers of the (so the oldest manuscripts read, not 'His') promise, in Christ Jesus (added in the oldest manuscripts), through the Gospel." It is "in Christ Jesus" that they are made "fellow heirs" in the inheritance of God: "of the same body" under the Head, Christ Jesus; and "fellow partakers of the promise" in the communion of THE Holy Spirit (Ep 1:13; He 6:4). The Trinity is thus alluded to, as often elsewhere in this Epistle (Ep 2:19, 20, 22).