Albert Barnes on 1 John 3:21

Beloved, if our heart condemn us not. If we so live as to have an approving conscience--that is, if we indulge in no secret sin; if we discharge faithfully every known duty; if we submit without murmuring to all the allotments of Divine Providence.

Then have we confidence toward God. Comp. 1Jn 3:19; 1Jn 2:28; Acts 24:16. The apostle evidently does not mean that we have confidence towards God on the ground of what we do, as if it were meritorious, or as if it constituted a claim to his favour; but that we may so live as to have evidence of personal piety, and that we may look forward with a confident hope that we shall be accepted of him in the great day. The word here rendered confidence--παρρησιαν--means properly boldness; usually boldness or openness in speaking our sentiments. 1Jn 2:28. The confidence or boldness which we have towards our Maker is founded solely on the evidence that he will graciously accept us as pardoned sinners; not in the belief that we deserve his favor.

(b) "heart" Job 27:6, Ps 101:2 (c) "confidence" Heb 10:22