Why should I believe that Christ is dwelling in Gentiles?
The mystery is that Christ is in a Gentile and is dwelling in Gentiles. You should believe it because that is what the book says. This truth was something a Jew could never comprehend, yet it is the secret to be acknowledged.
Les explains that the secret is that Christ is in a Gentile, something a Jew could never comprehend. He stresses that Christ is dwelling in Gentiles and says you had better believe it because that is what the book says. The passage also connects this to the acknowledgment of the mystery in Colossians 2:2 and to the request in Colossians 4:3 to speak the secret of Christ.