Jesus traced the Pharisees' words back to their hearts. They had just called the work of God's Spirit the work of the devil (Matthew 12:24, 28). Their words were the fruit, and the fruit showed the tree (Matthew 12:33). Barnes calls them "a wicked race; like poisonous reptiles, with a corrupt and evil nature." He adds that these "were strong expressions to be used by the meek and lowly Jesus; but they were not the effect of anger and malice." They were a true description. Gill explains that an evil man "may sometimes speak good things," but "these are not his common talk"; in the end "his speech betrays him."

Jesus answered His own question: "for out of the abundance of the heart the mouth speaketh" (Matthew 12:34). He went on: "A good man out of the good treasure of the heart bringeth forth good things: and an evil man out of the evil treasure bringeth forth evil things" (Matthew 12:35). He warned that "every idle word" will be accounted for in the day of judgment (Matthew 12:36-37). No reply to the charge itself is recorded. Instead, certain of the scribes and Pharisees "answered, saying, Master, we would see a sign from thee" (Matthew 12:38).

"Generation of vipers" translates gennemata echidnon, literally "offspring of vipers." Thayer says the phrase was applied to "cunning, malignant, wicked men." John the Baptist had used the same words for the Pharisees and Sadducees (Matthew 3:7), and Jesus would use them again (Matthew 23:33).

Gill notes the irony: "Though they boasted of their being the seed of Abraham," in character they were like vipers. Gill says that, like vipers, they hid poison, envy, and malice under an outward show of goodness.

This question shows that God looks at the heart, not just the words. The Pharisees were careful and religious on the outside, but their speech about Jesus revealed what was inside. Matthew Henry writes, "The heart is the fountain, words are the streams." If the fountain is bad, the streams will be too.

The hope in this passage is that God can change the source. Henry adds, "Nothing but the salt of grace, cast into the spring, will heal the waters." Jesus' words are severe, yet just before this He said that "all manner of sin and blasphemy shall be forgiven unto men," except blasphemy against the Holy Ghost (Matthew 12:31).