What does the Bible say about the heart and speech?

In Matthew 12:34-37, Jesus reveals that the words we speak reflect our inner heart condition. He speaks to the Pharisees, calling them a generation of vipers because, despite their outward religious appearances, they were filled with evil. This passage emphasizes that a good person, whose heart is transformed by grace, will utter good, truthful words that glorify God. Conversely, an evil person speaks out of a corrupt heart, demonstrating the necessity of a transformed nature for true goodness to manifest in our speech.