1-john 3:19

Hereby we know that we are of the truth - That we have the true religion of the Lord Jesus, and shall assure our hearts - be persuaded in our consciences, that we have the truth as it is in Jesus; as no man can impose upon himself by imagining he loves when he does not: he may make empty professions to others, but if he loves either God or man, he knows it because he feels it; and love unfelt is not love, it is word or tongue. This the apostle lays down as a test of a man's Christianity, and it is the strongest and most infallible test that can be given. He that loves feels that he does love; and he who feels that he loves God and man has true religion; and he who is careful to show the fruits of this love, in obedience to God and humane acts to man, gives others the fullest proof that he has the loving mind that was in Jesus.

And hereby - Greek, "by this;" that is, by the fact that we have true love to 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, that 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 Saviour. 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 toward 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. See Robinson, Lexicon, sub voce πείθω peithō, I. b. In other words, though a person'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.

3:19 And hereby we know - We have a farther proof by this real, operative love. That we are of the truth - That we have true faith, that we are true children of God. And shall assure our hearts before him - Shall enjoy the assurance of his favour, and the "testimony of a good conscience toward God." The heart, in St. John's language, is the conscience. The word conscience is not found in his writings.