How many times have you said that? Or how many times have you heard it said? It is true. The LORD does know your heart. But it doesn't let us off the hook. 1 Samuel 16.7 says
But the Lord said to Samuel, “Do not look at his appearance or at his physical stature, because I have refused him. For the Lord does not see as man sees; for man looks at the outward appearance, but the Lord looks at the heart.”
The LORD knows our inner thoughts, our deepest desires and our truest intentions. The LORD knows if they are pure or clouded by some other deep desire we have, but may not be aware of. God knows ALL that. The Bible says is Jeremiah 17.9 that the heart is deceitful. And Proverbs 21.2 says,
Every way of a man is right in his own eyes, But the LORD weighs the hearts.
Cain killed Abel because the LORD accepted Abel's sacrifice and not his. Surely God knew their deepest intentions and revealed them to them by accepting or not accepting their offering. In Psalm 51, David prayed,
Create in me a clean heart, O God,
And renew a steadfast spirit within me.
The LORD does know our hearts, even more than us. And the LORD has the only remedy to purify our clouded intentions and renew our hearts. Ask the LORD today, Are there areas in your life where your motives or intentions are driven by selfishness, anger, resentment, rebellion, hatred, lust or anything of the like? Be open to what is revealed to you, repent and believe that God will purify your heart, because of what Jesus did for us.