Hereby (en toutwi). See this phrase also in 2 Corinthians 2:5 ; 2 Corinthians 3:16 2 Corinthians 3:19 2 Corinthians 3:24 ; 2 Corinthians 4:22 Corinthians 4:13 ; 2 Corinthians 5:2 . That is explained by the ean clause, "if we keep his commandments " (ean thrwmen, condition of the third class, ean with present active subjunctive, "if we keep on keeping"), the clause itself in apposition with toutwi (locative case). Know we that we know him (ginoskomen oti egnwkamen auton). "Know we that we have come to know and still know him," egnwkamen the perfect active indicative of ginwskw. 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 (autou, Christ's) commandments, thoroughly Johannine phrase (12 times in the Gospel, 6 in this Epistle, 6 in the Apocalypse).