Theologians teach that the Bible's definition of the heart is "the seat of reflection." Therefore, we can examine the state of our heart by the decisions we make. Faulty decisions can come from a faulty heart, but they can also come from false beliefs about the real condition of our hearts as well. If we believe our hearts to be something other than they are, we're going to make a lot of faulty decisions in life because from our thoughts our actions follow.
Before we come to Christ, we do have a very serious heart condition. Scripture teaches that those who are still "in Adam" are going to display the realities of that family line. In order to have hope, lasting change, and peace, the fruits of the Spirit, etc--not to mention eternal life!--we must not simply be changed, or improved, we must be made brand new. We must become a part of a different family line.
Enter Christ Jesus.
When God sent His Son into the world, it wasn't just to teach us a better way to live. He could have used a prophet or even an angel to communicate a message like that. It wasn't to teach us how we can change our behaviors, He could have continued to send the Law. It wasn't even just to forgive us for all of our sins so we could go to heaven when we die. God could have established another way of getting folks to heaven. God sent Jesus to take our place for us as a man so that we could become a part of a different blood line through that sacrifice, through that atonement, and through our identification with Christ on His bloody cross. Only God could fulfill God's Holy Laws of perfection. Only God Himself could fix what man had broken. Only God would love the whole messy world enough to do so.
For those of us who are "in Christ," we no longer have ties to our Adam blood line. We weren't merely given a blood transfusion to empower us, we were given a complete heart transplant! We were made brand new! God has already changed us. We have a brand new identity. We have a brand new name. We have a brand new spirit. And now, it is His purpose and plan to mature us into who we already are. This new genetic identity allows us to reflect on events and circumstances differently, and by this, confirm to ourself and the world our new nature.
My friend, John Lynch, says this so much better than I ever could: