34 “Brood of vipers! How can you, being evil, speak good things? For out of the abundance of the heart the mouth speaks. 35 A good man out of the good treasure of his heart brings forth good things, and an evil man out of the evil treasure brings forth evil things.” (Matthew 12:34–35, NKJV)
Our heart is the source of our words, expressing what is in our heart. Jesus is quite emphatic that good words do not proceed from an evil heart. Conversely, a good heart does not speak evil things. We deceive ourselves if we think we can speak evil things and yet lay claim to having a good heart. So, we must “either make the tree good and its fruit good, or else make the tree bad and its fruit bad; for a tree is known by its fruit” (Matt. 12:33). Be careful of the words you speak. They define who you are. Your heart is known by your words, and by them you either will be justified or condemned in the day of judgment (Matt. 12:36-37).