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1 John 3:19 Updated American Standard Version (UASV)
19 By this we know that we are of the truth and will persuade our heart before him,
 Or assure; convince
And hereby. Gr., by this; that is, by the fact that we have true love for others and that we manifest it by a readiness to make sacrifices to do them good.
We know that we are of the truth. That we are not deceived in what we profess to be; that is, we are true Christians. To be of the truth stands opposed to cherishing false and delusive hopes.
And shall assure our hearts before him. Before God or before the Savior. In the margin, as in the Greek, the word rendered shall assure, is persuade. The Greek word is used as meaning to persuade, e.g., to the reception and belief of truth; then to persuade anyone who has unkind or prejudiced feelings towards us, or to bring over to kind feelings, to conciliate, and thus to pacify or quiet. The meaning here seems to be that we shall, in this way, allay the doubts and trouble of our minds and produce a state of quiet and peace, to wit, by the evidence that we are of the truth. Our consciences are often restless and troubled in view of past, guilt; but, in thus furnishing the evidence of true piety by love to others, we shall pacify an accusing mind, and conciliate our own hearts, and persuade or convince ourselves that we are truly the children of God. πείθω, I. b. In other words, though a man’s heart may condemn him as guilty, and though he knows that God sees and condemns the sins of his past life, yet the agitations and alarms of his mind may be calmed down and soothed by evidence that he is a child of God and that he will not be finally condemned. A true Christian does not attempt to conceal the fact that there is much for which his own heart and conscience might justly accuse him, but he finds, notwithstanding all this, evidence that he is a child of God, and he is persuaded that all will be well.
Persuasion: (Gr. πεισμονή peismonē, πείθω peithō) The Greek word literally means to 1.) persuade, convince (Matt. 27:20; Ac 12:20; 18:4; 19:8, 26; 23:28; 26:28). It means “to be assured of” or “to be convinced and certain of the truth of something.” Through the art of persuasion, one can cause another to adopt a certain position, view, belief, or course of action. Someone convinces or persuades another by bringing about a change of mind by means of sound, logical reasoning. Someone convinces or persuades another to adopt a new belief and to act on that belief. It also means to 2.) trust, rely (Lu 11:22; 2 Cor. 1:9); 3.) be assured (1 John 3:19); 4.) obey (Heb. 13:17); 5.) be a follower, be a disciple (Ac 5:36, 37); 6.) be certain, be sure (Heb. 13:18).
By Albert Barnes and Edward D. Andrews