For this reason I, Paul, a prisoner of Christ Jesus on behalf of you Gentiles— assuming that you have heard of the stewardship of God’s grace that was given to me for you, how the mystery was made known to me by revelation, as I have written briefly. When you read this, you can perceive my insight into the mystery of Christ, which was not made known to the sons of men in other generations as it has now been revealed to his holy apostles and prophets by the Spirit. This mystery is that the Gentiles are fellow heirs, members of the same body, and partakers of the promise in Christ Jesus through the gospel.

Ephesians 3:1-6

In the verses above Paul uses the word mystery to describe how the fulfillment of God’s plan through Christ’s sacrifice on the cross and His resurrection provides salvation for everyone who will call upon the name of the Lord (Romans 10:13), was not previously known.

The Jewish people were waiting for their messiah, their savior who would rescue them from the oppression they were facing from the Romans at the time. As a pharisee, Paul was likely of the same opinion, so the great mystery was revealed to Paul after his road to Damascus encounter with Christ.

Does mystery mean a truth can never be understood? I don’t think so. Paul explains the mystery plainly in the last sentence above –

“This mystery is that the Gentiles are fellow heirs, members of the same body, and partakers of the promise in Christ Jesus through the gospel.”

The Gospel is for everyone.