“Keep your heart with all vigilance, for from it flow the springs of life.” – Proverbs 4:23
“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. But to eat with unwashed hands does not defile anyone.” – Matthew 15:18-19
The Bible shows that the heart (aka, the inner man) is the source of life and death, good and evil. Unclean food doesn’t make one unclean; it’s what comes out of the heart. We are to guard the heart, because it’s the wellspring (source of) life.
We become unclean because we do not guard our hearts. We exude death because we have allowed sins to come in. Due to that allowance, death flows out of our heart through our mouth. When we allow impurity to come into the heart, it pollutes the well, which then flows out in our words. In short — garbage in, garbage out.