Mystery of Christ

Is the “mystery of Christ” in Ephesians 3:4 a mystery about Christ or is Christ Himself the mystery? Let’s take the latter option – Christ Himself is the mystery (cf. Colossians 1:27). How does that connect with the way Paul unpacks the mystery in terms of Jew/Gentile union (v. 6)? Through 1 Corinthians 12:12: Christ is the body, the body is Christ; and the mystery is that Jews and Gentiles together make up the body that with the head is the totus Christus.