Eph 3:6 This mystery is that the Gentiles are fellow heirs (a), members of the same body (b), and partakers of the promise in Christ Jesus through the gospel (c).
The second point the apostle makes is that the Gentiles are now members of the same body and in this body as we will see in chapter 4 have different gifts but one grace.
Rom 12:4-5 “4 For as in one body we have many members, and the members do not all have the same function, 5 so we, though many, are one body in Christ, and individually members one of another.”
The physical body is very complex yet each part works in perfect impeccable harmony where there are no anomalies, ailment, wounds and so on. The illustration of the members of the body sets forth the mutual reliance on one another as an image of members of the Church with their several gifts and capacities for the well-being of the whole body.
“So we, though many, are one body in Christ….” (Rom 12:5). This is the application of the above figure of speech. The chosen, redeemed of Christ, justified by His righteousness, and sanctified by His Spirit though few in comparison to the men of the world, are considered many (those with us are more than those against us). They make up but one body, the church, of which Christ is the head. The mystery of the gospel declares us as members of the same body and though many are one in Christ.