Why is the heart important in Christian theology?

In Christian theology, the heart represents the seat of our affections, thoughts, and will. Jesus pointed out in Matthew 12:34 that 'out of the abundance of the heart the mouth speaketh,' highlighting that our external conduct is a reflection of our internal state. The Bible asserts that the heart of man, in its natural state, is desperately wicked (Jeremiah 17:9), which necessitates a divine change. Through grace, God creates in believers a new heart that is capable of faith and obedience, enabling them to live in accordance with His will.