Heart Condemns Me Not

1Jo 3:20 — 1Jo 3:21 (KJV)
For if our heart condemn us, God is greater than our heart, and knoweth all things. Beloved, if our heart condemn us not, then have we confidence toward God.

So here, John is basically referring to putting our love for brethren in action (v,. 17,18). James echoes the same sentiments in James 2:15, 16 (KJV)
"If a brother or sister be naked, and destitute of daily food, And one of you say unto them, Depart in peace, be ye warmed and filled; notwithstanding ye give them not those things which are needful to the body; what doth it profit?"
Truly, even as we profess the name of Christ, as much shall we love our fellow brethren even as Christ laid down his life for us!

So the "heart" here is really about our conscience. Why is our conscience pricked (heart condemns) when we do not do what we are supposed to? Even if no one knows what's going on? Because God knows! Well did Jeremiah say in Jeremiah 23:24 (KJV)
"Can any hide himself in secret places that I shall not see him? saith the Lord. Do not I fill heaven and earth? saith the Lord."
But if we put our full confidence in God and His Word, we will truly and fully obey Him and that's when our conscience is truly clear.

The key then is to keep our hearts tender and sensitive to God's admonishments, and far be it for our conscience to be seared and be calloused and unfeeling (1Ti 4:2).