Jeremiah 17:9-10 (NKJV)
The heart is deceitful above all things, And desperately wicked; who can know it? I, the Lord, search the heart, I test the mind, Even to give every man according to his ways, According to the fruit of his doings.
When the Bible speaks of the heart, it is talking about the seat of a man's will and emotion. In this verse he speaks of "the" heart, not any particular heart. He's talking about your heart and my heart. Have you ever noticed how we are able to rationalize almost any decision or behavior? Oh, we criticize others but when it comes to our own actions, it seems we can defend almost anything. The scary thing is that we can convince our own selves.
You hear about wonderful, philanthropical gifts people make, and God bless them for their generosity. But God looks at the heart, not at the checkbook. You never know what the true motivation is. It may be just lo gain the praise of men.
When the Bible says the heart is deceitful above all things, it's true, things can't deceive; only people can. The Bible says it is a desperate and wicked situation. What to do? God does not solve the problem by cleaning up our old, dirty heart; He creates in us a new heart.
Psalm 51:10 (NKJV)
Create in me a clean heart, O God, And renew a steadfast spirit within me.