understand my knowledge - “perceive my understanding” [Alford], or “intelligence.” “When ye read,” implies that, deep as are the mysteries of this Epistle, the way for all to understand them is to read it (2Ti 3:15, 2Ti 3:16). By perceiving his understanding of the mysteries, they, too, will be enabled to understand.
the mystery of Christ - The “mystery” is Christ Himself, once hidden, but now revealed (Col 1:27).