And hereby we know that we are of the truth, and shall assure our hearts before him.
Hereby - `herein;' in loving in deed and truth (1 John 3:18).
We know. 'Aleph (') A B C have 'we shall know,' namely, if we fulfill the command (1 John 3:18).
Of the truth - real disciples of, belonging to, the truth, as it is in Jesus: begotten of God with the word of truth (James 1:18). Having the truth radically, we shall not love merely in word and tongue (1 John 3:18).
Assure - persuade, so as to cease to condemn us; satisfy the doubts of our consciences as to whether we be accepted before God or not (cf. Acts 12:20, "having made Blastus ... their friend," 'persuaded'). The "heart," the seat of the feelings, is our inward judge; the conscience, as witness, acts either as our justifying advocate, or our condemning accuser, even now. John nowhere (except John 8:9, rejected by the oldest manuscript) uses conscience. Peter and Paul alone use it.
Before him - in the sight of Him, the omniscient Searcher of hearts. Assurance is the designed experience and privilege of the believer.