Paul states in no uncertain terms that the "mystery of Christ" concerns itself with things concerning the Body of Christ:
"Whereby, when ye read, ye may understand my knowledge in the mystery of Christ) Which in other ages was not made known unto the sons of men, as it is now revealed unto his holy apostles and prophets by the Spirit; That the Gentiles should be fellowheirs, and of the same body, and partakers of his promise in Christ by the gospel" (Eph.3:4-6).
Do you really think that telling the unsaved that the Gentiles should be fellowheirs and of the same Body as the Jews will save anyone of them?
The mystery of the gospel of grace concerns the source of the prophesised grace and that source is in "bold" here, the fact that believers are
"justified freely by his grace through the redemption that is in Christ Jesus" (Ro.3:24).