Translation
[that] through the good news, the Gentiles are joint-heirs, and a joint-body,
Go to footnote numberand co-sharers,
Go to footnote numberof the promise [which is] in CHRIST JESUS,
Paraphrase
specifically that by believing and following the good news about Jesus, the non-Jews are co-heirs with the Jews of the benefits of following God, and form part of the same body of Christ along with the Jewish believers in Jesus, and are co-recipients of the positive consequences of the Promise of God which come to us in the person of THE SPIRIT ANOINTED MESSIAH who is GOD’S SAVIOR,
Footnotes
1: "joint-body":
in Greek this word is plural because it is talking about all the Gentiles, but English grammar requires that it to be singular because they have formed one body together with the Jewish believers.
2: co-sharers
comes from the preposition “with” and the word “sharer, partaker, recipient, partner.” It is quite similar to an heir; it is one who has a portion, or a share in something. For the heir it was usually land and other wealth, but this word could apply to things other than lands such as blessings, rights, privileges, food, protection, etc.
WHAT WAS THE MYSTERY?
The Biblical meaning of a mystery is something that has been concealed which is now being revealed by God. It is not like our mysteries which are simply things that are difficult to understand. These are part of God’s plan to unveil a body of knowledge when it is needed, not too early not too late.
The mystery that Paul was talking about was specifically that the Gentiles were intended to share as equals with the Jews in the blessings of salvation. It could not have been communicated more forcefully than “joint-heirs, a joint body and co-sharers.”
This was radical! This was unheard of! Although it was stated a few times in the Law and the prophets, this was one of those Biblical mysteries – it had been concealed or kept in an unclear condition, until it was fully revealed by God at the right time – through Jesus.
This is probably one of the things Paul went into the desert of Arabia to study and discern. Right after he got saved, he started preaching in Damascus, but at some point not long after that he became concerned about a burning question – “Is salvation for the Jews only or also for the Gentiles?” He could not continue preaching until he got an answer from the Lord on this matter.