John the Apostle
1 John 3:19WEB·traditional attribution

And by this we know that we are of the truth, and persuade our hearts before him,

Albert Barnes Presbyterian

Verse 19. And hereby. Gr., 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.

Geneva Bible Notes Reformed

And hereby we know that we are of the truth, and shall assure our hearts before him. (19) He commends charity, by three effects: for first of all, by it we know that we are indeed the sons of God, as he showed before.

John Calvin Reformed

19 And hereby we know, or, by this we know. The word truth, he takes now in a different sense; but there is a striking similarity in the words, — If we, in truth, love our neighbors, we have an evidence that we are born of God, who is truth, or that the truth of God dwells in us.