{Hereby} (en tout(9369)). See this phrase also in  2:5  3:16,19,24  4:2,13  5:2 . That is explained by the ean clause,
"if we keep his commandments " (ean t(8872)(936d)en, condition of the
third class, ean with present active subjunctive, "if we keep
on keeping"), the clause itself in apposition with 	out(9369)
(locative case). {Know we that we know him} (ginoskomen hoti
egn(936b)amen auton). "Know we that we have come to know and still
know him," egn(936b)amen the perfect active indicative of
gin(9373)k(935c). 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).