John Calvin Commentary 1 John 3:19

"Hereby shall we know that we are of the truth, and shall assure our heart before him:" — 1 John 3:19 (ASV)

And hereby we know, or, by this we know. He now uses the word truth in a different sense. However, there is a striking similarity in the words: if we, in truth, love our neighbors, we have evidence that we are born of God, who is truth, or that the truth of God dwells in us.

But we must always remember that we do not derive from love the knowledge the Apostle mentions, as though we were to seek the certainty of salvation from it.

And doubtless, we know that we are the children of God only as he seals his free adoption on our hearts by his own Spirit, and as we receive by faith the sure pledge of it offered in Christ. Then love is an accessory or an inferior aid, a prop to our faith, not a foundation on which it rests.

Why then does the Apostle say, We shall assure our hearts before God?

He reminds us by these words that faith does not exist without a good conscience. It is not that assurance arises from it or depends on it; rather, only then are we truly, and not falsely, assured of our union with God, when by the efficacy of his Holy Spirit he manifests himself in our love.

For it is always fitting and proper to consider what the Apostle is addressing. Since he condemns insincere and false professions of faith, he says that we cannot have a genuine assurance before God unless his Spirit produces in us the fruit of love.

Nevertheless, though a good conscience cannot be separated from faith, no one should therefore conclude that we must look to our works for our assurance to be certain.