The Mystery of Christ
When Paul uses the phrase "the mystery of Christ," he is not speaking about something vague or unknowable. He is referring to a specific revealed truth: that Gentiles are fellow heirs with Israel, members of the same body, and sharers in the promise through the gospel. This is one of the most developed uses of mystērion in the entire New Testament, and it appears most prominently in Ephesians and Colossians.
The Key Passage: Ephesians 3:3-6
"...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 3:3-6 (ESV)
Here Paul spells out the content of the mystery with unusual clarity. The mystery is Gentile inclusion — not merely that Gentiles could be saved (which the Old Testament anticipated in places like Isaiah 49:6), but that they would be incorporated into the same body on equal footing, without first becoming Jewish proselytes.
Colossians: "Christ in You, the Hope of Glory"
In Colossians, Paul approaches the same mystery from a slightly different angle:
"To them God chose to make known how great among the Gentiles are the riches of the glory of this mystery, which is Christ in you, the hope of glory."— Colossians 1:27 (ESV)
Here the mystery is described as "Christ in you" — the indwelling presence of Christ among Gentile believers. In Colossians 2:2, Paul says his goal is that believers would reach "the knowledge of God's mystery, which is Christ," identifying Christ Himself as the content and embodiment of the mystery. In Colossians 4:3, he asks for prayer that "God may open to us a door for the word, to declare the mystery of Christ."
Why Was This a Mystery?
The inclusion of Gentiles was not entirely unanticipated in the Old Testament. Passages like Genesis 12:3 ("in you all the families of the earth shall be blessed"), Isaiah 42:6 ("a light for the nations"), and Psalm 87 hint at a wider scope for God's saving purposes. But the specific manner of inclusion — Gentiles being made fellow heirs in the same body, through faith alone, without the Law — was not clearly perceived until it was revealed through the apostles.
Paul emphasizes this in Ephesians 3:5: the mystery "was not made known to the sons of men in other generations as it has now been revealed." The word "as" is important. It does not say the mystery was completely unknown before, only that it was not known in the way it has now been revealed. The full picture required the Christ event to bring it into focus.
For more on this timeline of concealment and revelation, see The Mystery Hidden for Ages.
The Mystery of Christ and the Church
The mystery of Christ is closely related to the mystery of marriage in Ephesians 5:32, where Paul says that the union of husband and wife is a picture of Christ and the church. The "body" language in Ephesians 3 (Gentiles as members of the same body) and the "bride" language in Ephesians 5 are two expressions of the same fundamental mystery: that Christ and His people are united.
Relationship to the Mystery of the Gospel
The mystery of Christ and the mystery of the gospel are closely related but not identical. The mystery of the gospel refers more broadly to the message of salvation through Christ; the mystery of Christ focuses specifically on the composition of God's people — who is included and on what basis. Both are aspects of the larger revealed plan described in the mystery of God's will (Ephesians 1:9-10).