But what comes out of the mouth proceeds from the heart, and this defiles a person. For out of the heart come evil thoughts, murder, adultery, sexual immorality, theft, false witness, slander. These are what defile a person.
This is the core teaching of Christ that gets to me. I know it to be real. I know that it is what is inside my heart that is most evil. And the sin there is what I cannot seem to control, no matter how much I resent it being there. Evil crops up and comes out. And it comes from the heart.
The Pharisees and Jewish authorities of Jesus' day had external religion tied down. They pushed so hard for conformance to rules. But they failed miserably at transformation of the heart. And that is where Jesus excels. He gets to the root issues by insisting that the kind of change He brings is one of the heart.
I personally know that is where the Christian life begins and grows in me. It is at the heart. It is internal. It is a war inside of me. And Jesus brings victory and not only defines the struggle, but offers hope beyond it.