John Calvin Reformed @genevareformer

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.

Matthew Henry Presbyterian @wholebiblehenry

The beloved apostle can scarcely touch upon the mention of sacred love, but he must enlarge upon the enforcement of it, as here he does by divers arguments and incentives thereto; as, I. That it is a mark of our evangelical justification, of our transition into a state of life: We know that we have passed from death to life, because we love the brethren, Jo1 3:14.

Commenting on 1 John 3:14-19

John Gill Reformed Baptist @doctorgill

For if our heart condemn us,.... Of want of love to the brethren, and of hypocrisy in it, as well as of any other sin; for the conscience, which is meant by the heart here, is accuser, witness and judge; it accuses of the evil of sin, and is as good as a thousand witnesses; and upon its own testimony pronounces guilty, and condemns.