Jamieson, Fausset and Brown on 1 John 2:3

3. hereby--Greek, "in this." "It is herein," and herein only, that we know (present tense) that we have knowledge of (perfect tense, once-for-all obtained and continuing knowledge of) Him" (1Jo 2:4, 13, 14). Tokens whereby to discern grace are frequently given in this Epistle. The Gnostics, by the Spirit's prescient forewarning, are refuted, who boasted of knowledge, but set aside obedience. "Know Him," namely, as "the righteous" (1Jo 2:1, 29); our "Advocate and Intercessor."

keep--John's favorite word, instead of "do," literally, "watch," "guard," and "keep safe" as a precious thing; observing so as to keep. So Christ Himself. Not faultless conformity, but hearty acceptance of, and willing subjection to, God's whole revealed will, is meant.

commandments--injunctions of faith, love, and obedience. John never uses "the law" to express the rule of Christian obedience: he uses it as the Mosaic law.