In Matthew 12:34 Jesus applies this phrase to the Pharisees to show how their words reveal their inner reality. Their inward corruption, means that they can not say anything truly good. Whatever fills the heart overflows through the lips; speech exposes what the heart is full of. This phrase also shows that the Pharisees are evil in seeking to escape God’s wrath without true repentance through John’s baptism, in contrast to those who, through John’s ministry, had come to expect the Messiah. John’s preaching showed the need to confess Jesus. Those unwilling to repent, however, speak only words of rejection and blasphemy out of their hardened unbelief. The image is like the tree and its fruit, as words show the state of a person’s heart. The heart, like a reservoir of thoughts, overflows in words (see James 3:11–12). The Pharisees’ hostile words show the presence of an anti-divine nature at work within them
In Matthew 12:34 Jesus applies this phrase to the Pharisees to show that their words reveal their inner reality. Because they are inwardly corrupt, they are incapable of speaking anything truly good. Whatever fills the heart in abundance inevitably overflows through the lips; speech exposes what the heart is full of.1
With this phrase Jesus highlights that the Pharisees are evil because they seek to escape God’s wrath without true repentance through John’s baptism. They stand in contrast to those who, through John’s ministry, had come to expect the Messiah. From John’s faithful preaching, such people learn the need to confess Jesus. Those unwilling to repent, however, can draw only words of rejection and blasphemy out of their hardened unbelief and unwillingness.
The image is similar to a tree and its fruit: just as fruit shows the nature of the tree, so words disclose the state of a person’s heart. The heart is like a reservoir of thoughts, and spoken words are its overflow (see James 3:11–12). In the Pharisees’ case, their hostile words demonstrate the presence of an anti-divine nature already firmly at work within them.2
34 You brood of vipers! How can you speak good, when you are evil? For out of the abundance of the heart the mouth speaks.