Jeremiah 17:9. The heart is deceitful above all things, and desperately wicked: who can know it?
Heart-evils are very provoking evils to the Lord. The Schools correctly observe, that outward sins are “sins of great infamy;” but that the heart sins are “sins of deeper guilt.” How severely has the great God declared wrath from heaven against heart-wickedness! The crime of the old world stands indicted is heart-wickedness!” “God saw that every imagination of their hearts was only evil continually;” for which he sent the most dreadful judgments that were ever inflicted since time began. We find not their murders, adulteries, blasphemies, (though they were defiled with these) particularly alleged against them; but the evils of their hearts. Keeping the Heart – John Flavel
Like the Pharisees, we put the sins of the flesh as being worse than the sins of the heart. God’s priority is to keep the heart first. It is a lot easier to hide the sins of the heart from those around us, so we tend to keep it at a low priority because we try to impress man.
However, you cannot hide your heart from God. The wicked thoughts that go through our minds come from a wicked heart. As Flavel says, heart sins are sins of deeper guilt. It was in the days of Noah that hearts would only think continually about evil. This is what brought the judgment of the flood on the earth. The heart is the source of evil and that is why it is so important to God.
God wants you to keep your heart with all diligence. It is the one thing you should work on most. If you have a right heart, you will be a good witness. If you have a right heart, you will love the brethren. If you have a right heart, you will have peace and joy. You need to keep your heart right with God. Keeping your thoughts pure and not thinking about vain, empty, worthless things that will not matter in eternity is vital. The mind and heart go hand in hand. You need to fight tooth and nail to think right so your heart is right. The Lord values it.