1 John 2:3

Hereby

(εν τουτω). See this phrase also in 1 John 2:5; 1 John 3:16; 1 John 3:19; 1 John 3:24; 1 John 4:2; 1 John 4:13; 1 John 5:2. That is explained by the εαν clause, "if we keep his commandments " (εαν τηρωμεν, condition of the third class, εαν with present active subjunctive, "if we keep on keeping"), the clause itself in apposition with τουτω (locative case).Know we that we know him

(γινοσκομεν οτ εγνωκαμεν αυτον). "Know we that we have come to know and still know him," εγνωκαμεν the perfect active indicative of γινωσκω. The Gnostics boasted of their superior knowledge of Christ, and John here challenges their boast by an appeal to experimental knowledge of Christ which is shown by keeping his (αυτου, Christ's) commandments, thoroughly Johannine phrase (12 times in the Gospel, 6 in this Epistle, 6 in the Apocalypse).