hereby - Greek, “herein”; in our loving in deed and in truth (1Jo 3:18).
we know - The oldest manuscripts have “we shall know,” namely, if we fulfil the command (1Jo 3:18).
of the truth - that we are real disciples of, and belonging to, the truth, as it is in Jesus: begotten of God with the word of truth. Having herein the truth radically, we shall be sure not to love merely in word and tongue. (1Jo 3:18).
assure - literally, “persuade,” namely, so as to cease to condemn us; satisfy the questionings and doubts of our consciences as to whether we be accepted before God or not (compare Mat 28:14; Act 12:20, “having made Blastus their friend,” literally, “persuaded”). The “heart,” as the seat of the feelings, is our inward judge; the conscience, as the witness, acts either as our justifying advocate, or our condemning accuser, before God even now. Joh 8:9, has “conscience,” but the passage is omitted in most old manuscripts. John nowhere else uses the term “conscience.” Peter and Paul alone use it.
before him - as in the sight of Him, the omniscient Searcher of hearts. Assurance is designed to be the ordinary experience and privilege of the believer.