Either make the tree good, and his fruit good; or else make the tree corrupt, and his fruit corrupt: for the tree is known by [his] fruit. O generation of vipers, how can ye, being evil, speak good things? for out of the abundance of the heart the mouth speaketh. 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. But I say unto you, That every idle word that men shall speak, they shall give account thereof in the day of judgment. For by thy words thou shalt be justified, and by thy words thou shalt be condemned.Matthew 12:33-37
An apple tree does not become an apple tree when it bears the fruit of apples. The apple tree, by its nature, bears apple fruit. And so it is with good and evil works and words.
This example is often used to show that wicked works a person does shows that the person is wicked. His actions do not define who he is but rather shows his nature. In this case, Jesus is not necessarily talking about the nature of people.
He is referring to the motivation of a works and words.
The Pharisees said that Jesus did a good thing, casting out devils, by an evil motivation, the power of the devil. This is completely illogical.
If you declare an action to be good, then you must declare that the motivation and power behind are also good. The same goes for evil actions.
Then, Jesus turns the tables on the Pharisees. He calls them a “generation of vipers.” He flat out calls them evil, and that it is therefore impossible for them to say any good thing.
Let us be reminded that we do not have the knowledge, power, or authority to make such claims about other people. This lesson is in scripture to prove to us that Jesus is the Messiah. He has the right and ability to make such declarations because He is the Son of God.
The entire teaching regarding blasphemy against the Holy Ghost found in Matthew 12 and Mark 3 should at least give us pause to recognize how serious a crime it is to attribute the works of God to the power of Satan!