Vincent's NT Word Studies
4. Whereby (prov o). Lit., agreeably to which, namely, what he had written.Mystery of Christ. The mystery which is Christ. See on Col. i. 26; Rom. xi. 25.
Robertson's NT Word Studies
3:4 {Whereby} (pros ho). "Looking to which,"according to which." {When ye read} (anaginwskontes). this epistle will be read in public. {My understanding in the mystery of Christ} (ten sunesin mou en twi musteriwi tou cristou). My "comprehension" (sunesin, #Col 1:9; 2:2). Every sermon reveals the preacher's grasp of "the mystery of Christ." If he has no insight into Christ, he has no call to preach.