1 John 3:19

“And hereby we know that we are of the truth, &c.] By the saints loving one another in deed and in truth, they know, as the cause is known by the effect, that they are of God, who is the true God, the God of truth, and cannot lie, and is truth itself; that they are the children of God, and are born of him, since they love those that are, and every like loves its like; and that they are of Jesus Christ, who is the way, the truth, and the life; that they belong to him, are his, since they have his Spirit, as appears by his fruits in them, and this, among the rest, love to the brethren; and that they are his disciples, which others, even all men know, as well as themselves, by their mutual…”

“hereby--Greek, "herein"; in our loving in deed and in truth (Jo1 3:18). we know--The oldest manuscripts have "we shall know," namely, if we fulfil the command (Jo1 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. (Jo1 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…”

“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…”

“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. 1 John 3:20”

“3:19-22 Those who truly love (3:16-18) have confidence that God accepts them because real love is primary evidence of real faith and of new life in Christ (see 4:7-8).”

“hereby: Jo1 3:14, Jo1 1:8; Joh 13:35, Joh 18:37 shall: Jo1 3:21; Isa 32:17; Heb 6:10, Heb 6:11, Heb 10:22 assure: Gr. persuade, Rom 4:21, Rom 8:38; Ti2 1:12; Heb 11:13 1 John 3:20”