Some passages of scripture are difficult to translate and understand. 1 John 3:19-20 is one of those passages. Look at the various ways translators have wrestled with these two verses. (I’ve underlined some distinctive features).
“By this we shall know that we are of the truth and reassure our heart before him; for whenever our heart condemns us, God is greater than our heart, and he knows everything.” ESV
“This is how we know that we belong to the truth and how we set our hearts at rest in his presence: If our hearts condemn us, we know that God is greater than our hearts, and he knows everything.” NIV
“We will know by this that we are of the truth, and will assure our heart before Him in whatever our heart condemns us; for God is greater than our heart and knows all things.” NASB
“My dear children, let’s not just talk about love; let’s practice real love. This is the only way we’ll know we’re living truly, living in God’s reality. It’s also the way to shut down debilitating self-criticism, even when there is something to it. For God is greater than our worried hearts and knows more about us than we do ourselves.” MSG
What should we make of these various options? The Expositor’s Bible Commentary describes the variations we’ve just seen above and suggests a preferred interpretation worth considering:
“The passage itself is complex in the Greek and allows several translations and interpretations. “This” (v.19) may be taken to point backward to the absolute demand of love introduced in 3:14ff. If we know that we love truly, with actions and not mere words, that knowledge will not only assure us “that we belong to the truth” but will also act to “set our hearts at rest in his presence whenever our hearts condemn us.”
It is possible, however, in the Greek to make a full stop after “presence” and then read v.20 as follows: “If our hearts should condemn us, God is greater than our hearts, and he knows everything.”
Another possibility is that the “this” in v. 19 points not only backward to 3:14ff. but forward to v. 20b. the meaning would be as follows: there are two ways we know that we “belong to the truth”: First, because we love in deed; second, God himself assures us that we belong to the truth–he “is greater than our hearts, and he knows everything.” The latter possibility is preferable because it allows a more connected argument.” (The Expositor’s Bible Commentary, p.337)
In other words, this final interpretation should be preferred because it not only makes sense of the Greek, but it also allows for a continuation of the logic and flow of thought in the immediate context.