Whereby, when ye read, ye may understand my knowledge in the mystery of Christ)
Whereby - `according to which,' writing in brief (Ephesians 3:3).
Understand my knowledge, х noeesai (G3539) sunesin (G4907), 'observe my insight:' sunienai (G4920), to comprehend; noein, to perceive, observe, notice]. Deep as are the mysteries of this letter, the way to understand them is to read it (2 Timothy 3:15-16). By observing his comprehension of the mysteries, they, too, will be enabled to comprehend.
The mystery of Christ - i:e., Christ Himself, once hidden, but now revealed as reconciling Jew and Gentile to God, and so to one another (Colossians 1:27).