A mystery is something that puzzles or perplexes people. It is something that does not immediately make sense when you hear it. The mystery of Christ as it is referred to in Eph 3 refers to the grace if God. The perplexity it causes is largely due tot the fact that it just sounds too good to be true.
To the Jews it didn’t make sense that Gentiles could now be includes in God’s special promise through simple acceptance of the Gospel. Jesus shook all their ideas. Now it is no more a matter of observing the law and commandments (which only included the Jews), but now it is a matter of belief (which includes all people and races on earth).
“And that the mystery (secret) was made known to me…when you read this you can understand my insight into the mystery of Christ. This mystery was never disclosed to human beings in past generations…[It is this:] that the Gentiles are now to be fellow heirs [with the Jews], members of the same body and joint partakers [sharing] in the same divine promise in Christ through [their acceptance of] the glad tidings (the Gospel).” Eph 3:3-6
The grace of Jesus is called a favor or privilege, showing to us the unending, boundless, fathomless, incalculable and exhaustless riches of Christ. It is a wealth which no human being could have searched out (Eph 3:8). It includes every believer on earth, regardless of his race, performance, religious background or influence. It is easy and simple – not dependant on our discipline or achievements. We only need to believe in order to share in these riches.