And hereby we do know that we know him, if we keep his commandments.
Hereby, х en (G1722) touto (G5124)] - 'in this.' 'It is herein only that we know (present) that we have knowledge of (perfect: once-for-all obtained and continuing knowledge of) Him' (1 John 2:4; 1 John 2:13-14). Tokens whereby to discern grace are frequent in this letter. The Gnostics, by the Spirit's prescient forewarning, are refuted, who boasted of knowledge, but set aside obedience. "Know Him," namely, as "the righteous" (1 John 2:1; 1 John 2:29): our "Advocate and Intercessor."
Keep - John's favourite word, instead of do х teeroomen (G5083)], to guard, and keep safe as a precious thing; observing so as to keep. So Christ Himself (John 15:10). Not faultless conformity, but hearty acceptance of, and willing subjection to, God's whole revealed will.
Commandments, х entolas (G1785)] - injunctions of faith, love, and obedience. John uses 'the law' to express, not the rule of Christian obedience, but the Mosaic law.