Rev. witness. “He seems to be alluding to his conduct of former days, and to say, 'I know something of it, of that zeal'” (Godet).
Zeal of God (ζῆλον Θεοῦ)
Rev., zeal for God. Like the phrase “faith of Christ” for “faith in Christ” (Phi 3:9); compare Col 2:12; Eph 3:12; Joh 2:17, “the zeal of thine house,” i.e., “for thy house.”
Knowledge (ἐπίγνωσιν)
Full or correct and vital knowledge. See on Rom 1:28; see on Rom 3:20.