No good tree bears bad fruit, nor does a bad tree bear good fruit. Each tree is recognized by its own fruit. People do not pick figs from thornbushes, or grapes from briers. A good man brings good things out of the good stored up in his heart, and an evil man brings evil things out of the evil stored up in his heart. For out of the overflow of his heart his mouth speaks.

–Parallel verses:
Matthew‬ ‭15‬:‭16‬-‭20‬
“Are you still so dull?” Jesus asked them. “Don’t you see that whatever enters the mouth goes into the stomach and then out of the body? But the things that come out of a person’s mouth come from the heart, and these defile them. For out of the heart come evil thoughts—murder, adultery, sexual immorality, theft, false testimony, slander. These are what defile a person; but eating with unwashed hands does not defile them.”

Input … output. A good fruit tree has good soil, good nutrients, and a strong root system. A bad tree might be starved of nutrients, have a disease, etc. So, Jesus is using a tree’s root system as a metaphor for a person’s heart. And in the parallel passage in Matthew, Jesus explains that the heart of man is full of sin.

So how on earth can we change our hearts? The only way we can change our hearts is to change the one who runs it and is in control of its inputs, or nutrients. So, as I start my day I have a choice to make: Who will be in control of my heart? And throughout the day I choose what nutrients or inputs into my heart I will take in. Will they be full of worldly desires and pleasures? Or will I fill my mind and heart with worship, praise, prayer, thanksgiving, and love. Whatever I fill my heart with, will come out in my speech and actions.

Lord, teach me to crave and hunger and thirst the things of you each day. May I dine on your word and meditate on your promises. You alone are the food my heart needs to be clean. Purify me that I may be a holy vessel for use in your hands.