Matthew 7:15-7:20 “Watch out for false prophets. They come to you in sheep’s clothing, but inwardly they are ferocious wolves. By their fruit you will recognize them. Do people pick grapes from thornbushes, or figs from thistles? Likewise every good tree bears good fruit, but a bad tree bears bad fruit. A good tree cannot bear bad fruit, and a bad tree cannot bear good fruit. Every tree that does not bear good fruit is cut down and thrown into the fire. Thus, by their fruit you will recognize them.”
In this passage, Jesus warns against false prophets by saying that they’re known by the bad products they bring, i.e. their “bad fruit.” Nothing is mentioned about what standard is used to judge “good fruit” from “bad fruit,” but one can infer that, since it’s unlikely Jesus is referring to himself when he mentions false prophets, “good fruit” are like the teachings and miracles of Jesus while “bad fruit” are like the teachings of the pharisees and pagans. Jesus uses the analogy of fruit and thorns to explain what “good trees” and “bad trees” are, which brings a secular standard of life as the analogous standard to holiness, that life-affirming fruit is like God-affirming behavior. This creates a situation where Jesus’ analogy can be evaluated as general wisdom. Even if he is ultimately talking about other-worldly standards (the standard of the afterlife and God), his propositions can be judged by their merit in how accurately they portray the real world. In terms of his comparison of people and fruit, there’s some logical sense in what he says, and there are cases that his analogy doesn’t quite fit.
The passage certainly makes sense in some way. One would expect bad fruit from a bad tree and good fruit from a good tree. The idea is that the tree is the root from where the fruit comes from, and the quality of that root determines the quality of the fruit. Jesus brings up thorns, grapes, and figs to illustrate the point. A thornbush creates thorns, of which people don’t want, and a grape vine creates grapes, of which people want. In the example, the type of “tree” it is determines the type of “fruit.” Thinking of things in terms of roots that have subsequent causes can be a realistic approach. The root of me taking a diligent interest in mathematics leads to the “good fruit” of correct math answers. Who you are ultimately affects what you do.
However, Jesus says that good trees only bear good fruit and bad trees only bear bad fruit, which doesn’t fit with the reality of people. There’s no guarantee that a good person always bears good fruit nor an evil person always bears bad fruit. For example, a person can be a good person but still make a mistake or have bad proclivities (a good person could have an eating disorder or something similar). An evil person may be typically evil, but he/she still needs to perform certain good acts to get by (serial killers are often seen as normal, helpful, productive people until they are found out). If one were to believe Jesus, one would have to assume that as soon as someone saw someone do a good or bad thing they would automatically be good or bad, respectively. Real people are more mixed, however. They have developed their habits over a long period of time and influences, and they don’t always evaluate everything that comes into their mind. One cannot expect a world of people that are perfectly good or perfectly evil, whatever that would be. In the analogy of the tree, one would have to expect that every apple on an apple tree is either good or bad, but the apples of each tree have varying qualities.
In this passage, Jesus makes some sense, but he ultimately makes too sweeping of a generalization. One can expect good fruit from a good tree and bad fruit from a bad tree, but it doesn’t follow that you would only get good fruit from a good tree and bad fruit from a bad tree. To assume so would put one in the state of thinking someone is good or evil based on a single isolated act (as long as you saw someone do something good, you would assume that person was totally good, as the same with evil). One should expect good things from good people and evil things from evil people, but one wouldn’t assume a total purity of action in regards to either.