If you make the tree good, its fruit will also be good. if you make the tree bad, its fruit will also be bad, because a tree is known by its fruit.
In this saying, Jesus uses an illustrative example about a tree and its fruit to explain the connection between a person’s inner nature and outward deeds. Jesus’ words emphasize that a person’s actions directly express the condition of the heart. Jesus speaks these words to the Pharisees who were accusing Him, while their own lives were contrary to God’s truth. By this example, Jesus shows that true faith and inner goodness inevitably are expressed through good works, while an evil heart gives birth to evil deeds. This saying is also a call to self-examination: a person must examine his “fruits”—his actions and his life—in order to understand the condition of his heart. If the fruits are not good, then the nature of the tree must be changed—the person’s inner life—which is possible only by God’s grace. In summary, Jesus emphasizes that a person’s inner nature is known by his deeds, and only a truly good heart can produce truly good fruit.