What does the Bible say about the heart and words?

According to Matthew 15:16-19, Jesus explains that it is not what enters a person's mouth that defiles them, but what comes out, as it reflects the true condition of their heart. This concept is foundational in understanding that all humans, by nature, have sinful hearts that produce sinful speech. Proverbs 18:4 reiterates this by stating that the words of a man's mouth are as deep waters, indicating that our speech reveals the depths of our character and the state of our hearts. Isaiah 57:20 further illustrates this, describing the wicked as troubled seas that cast up mire and dirt, symbolizing the corruption within our hearts.