Matthew 12:33-37 meaning

This particular teaching is unparalleled in the gospel accounts, though it is similar in many regards to Matthew 7:16-20.

The Pharisees had just spoken against the Holy Spirit. And after Jesus finished pointing out the seriousness of this blasphemy, He rebukes them for the evil in their hearts and the carelessness of their words. To make His point and show the relationship between a man's heart and his words, Jesus draws an analogy. Either make the tree good and its fruit good, or make the tree bad and its fruit bad; for the tree is known by its fruit (v 33).

 The tree Jesus describes is a fruit tree. Each tree is known by the kind and quality of the fruit it produces. If the tree produces good fruit the tree is good (v 33). If the tree produces bad fruit the tree is bad (v 33). But the tree is the producer of and is therefore the cause of the fruit, not the other way around. We either praise or blame the tree for the quality of its fruit. If someone desires good fruit, he will have to first make the tree good. It would be futile to try to make the tree good by making its bad fruit better. In this analogy, the tree is a man's heart, and the analog of the tree's fruit are the words that man's mouth speaks. In order to speak good words, the heart must be first made good.

After this analogy Jesus rebukes the Pharisees. He addresses them, You brood of vipers. This was the same label that John the Baptist called the Pharisees and Sadducees when they went out into the wilderness to see him (Matthew 3:7). Vipers are venomous snakes. Vipers are associated with evil and the devil, because the serpent deceived Eve into disobeying God. In calling the Pharisees a brood of vipers, Jesus is calling them spawns of the devil. There is an irony in Jesus's description. The Pharisees had just accused Jesus of working in the power of Beezlebub, a nickname for Satan (Matthew 12:24), and now Jesus is calling the Pharisees a brood of vipers, offspring of the Serpent.

Jesus asks them a rhetorical question, How can you, being evil, speak what is good? (v 34). Because they are evil, they are unable to speak what is good. They are bad trees with bad fruit.

Jesus then states the principle of His fruit tree analogy. For the mouth speaks out of that which fills the heart (v 34). Just as fruit is the produce of the tree, the words that proceed from the mouth of a man are the produce of the heart of that man. Jesus very well may have had Proverbs 10:11 in mind as He said these things to the Pharisees.

 "The mouth of the righteous is a fountain of life,
But the mouth of the wicked conceals violence."
(Proverbs 10:11)

 Jesus elaborates and explains his fruit tree analogy and proverb with another analogy. The good man brings out of his good treasure what is good; and the evil man brings out of his evil treasure what is evil (v 35).

Jesus's remarks are reminiscent of what He taught about false prophets in His Sermon on the Mount.

"You will know them by their fruits. Grapes are not gathered from thorn bushes nor figs from thistles, are they? So every good tree bears good fruit, but the bad tree bears bad fruit. A good tree cannot produce bad fruit, nor can a bad tree produce good fruit. Every tree that does not bear good fruit is cut down and thrown into the fire. So then, you will know them by their fruits."
(Matthew 7:16-20)

Jesus then warns the Pharisees to watch what they say. This is because every careless word that people speak, they shall give an accounting for it in the day of judgment (v 36). Just as will all humanity, Pharisees are accountable for the things they say, even in careless moments. There are careless words in every remark we make, every curse we utter, every lousy joke we tell, every comment we post and later delete on social media, every word we say reveals something of our heart. And our own words will either justify or condemn us in the day of judgment.

"If you say, 'See, we did not know this,'
Does He who weighs the hearts not consider it?
And does He who watches over your soul not know it?
And will He not repay a person according to his work?"
(Proverbs 24:12)

But Jesus's point in saying all of this to the Pharisees is not for them to merely watch their words; it is much more important for them to guard their hearts. Our words are a key indicator of what is in our hearts:

 "Watch over your heart with all diligence,
For from it flow the springs of life."
(Proverbs 4:23)