Proverbs 4:23-27 (NIV)
 Above all else, guard your heart,
for it is the wellspring of life.
 Put away perversity from your mouth;
keep corrupt talk far from your lips.
 Let your eyes look straight ahead,
fix your gaze directly before you.
 Make level paths for your feet
and take only ways that are firm.
 Do not swerve to the right or the left;
keep your foot from evil.
These verses are crucial to understanding life. In them God informs us that people and situations are not the controlling factor in our words and actions. God informs us that our heart directs our behavior whether in word or deed. In Luke’s Gospel (Luke 6:43-45) Jesus speaks to the truth of Proverbs 4:23 by using the metaphor of trees and fruit.
Luke 6:43-46 (NIV)
 “No good tree bears bad fruit, nor does a bad tree bear good fruit.  Each tree is recognized by its own fruit. People do not pick figs from thornbushes, or grapes from briers.  The good man brings good things out of the good stored up in his heart, and the evil man brings evil things out of the evil stored up in his heart. For out of the overflow of his heart his mouth speaks.
Just as there’s a systemic and organic relationship between a tree and it’s fruit — there is a systemic and organic relationship between my heart and my life; my heart and my behavior! Sin is not rooted outside of me. Circumstances and relationships are merely the location that the sin in my heart is revealed.
What should we do? If the heart is the source of sinful behavior, any genuine and lasting change must begin with my heart and work its way out into the fruit of my life. If my heart isn’t changed, any change in my words and actions will only be the result of some external pressure (If I don’t change I’ll lose my marriage, my girlfriend or my job). That kind of change is only temporary. When the external pressure is gone, or if my incentive to change goes away (the spouse leaves, the job is terminated) the apparent change vanishes.
That’s why any transformation of behavior passes through the heart.
This is the core difference between religion and authentic Christianity. Religion imposes a set of external religious/moral demands upon men and women that are supposed to change the heart. Christianity is all about Jesus radically transforming the hearts of men and women — God giving to us new life — God working in us a desire to do His will and the power to do it — producing the fruit of a radical new set of values and way of living.
The hard truth — If you’re careless with your heart, your life will get messy. We see time and time again throughout the Bible that the relationship between a person's heart and their words, their actions and their life are examined.
Paul tells the believers in Corinth, "Examine yourselves to see whether you are in the faith; test yourselves. Do you not realize that Christ Jesus is in you—unless, of course, you fail the test?" 2 Corinthians 13:5 (NIV)
What is he asking us to examine? It's our heart. I need to make sure that Jesus is still at the center of my heart therefore of my words and deeds.
Colossians 3:17 (NIV)
 And whatever you do, whether in word or deed, do it all in the name of the Lord Jesus, giving thanks to God the Father through him.