Verses 15–20 present examples of how believers can discern false prophets (v. 15). The existence of this passage predicts the rise of such false prophets, or discerning them would be an unnecessary skill. This information is appropriate for the Old World where apostasy was predicted. Indeed, we need look no further than Paul’s letters to see that the churches he established drifted from correct teachings not long after he had taught the true gospel to them. The first century after Christ saw an explosion of Christianity but also tremendous pressures on Christian communities. The Didaché, a Christian manual dating from perhaps A.D. 110, describes a community of believers who accepted itinerant teachers, preachers, apostles, and prophets as they came through. Among these were also the good and the bad, and that document contains many hints on how to tell them apart. (See commentary accompanying 3 Nephi 14:21–23.)
Jesus uses a botanical metaphor as ultimate proof of a person’s true nature. The plant’s invisible essence manifests itself the visible fruit. Thus, it is impossible to gather “grapes of thorns, or figs of thistles.” We can receive a given fruit only from the plant whose nature is to produce that fruit. In this case, we can have good fruit only from a good plant.
There are, of course, plants that ought to be good that still do not produce good fruit. We do not expect figs of thorns, but we do expect grapes of a grapevine. Nevertheless, some vines do not produce good fruit. The entire olive allegory in Jacob depends upon the fact that some fruit is not good, regardless of the type of tree.
Book of Mormon Context: Like the sayings about roadways, this metaphor would have had limited usefulness to the Nephites. Despite some experiences with false prophets (e.g., Nehor and Korihor), they were not typical nor would the next two hundred years provide a situation like the Old World’s. The Nephites were the dominant regional religion, not a minority like the Christians in the Old World. Furthermore, they maintained faithfulness for about two hundred years instead of suffering the types of widespread apostasy that were part of the Old World situation. This key of discernment lacked the immediacy of its Old World context.
Reference: Andrew C. Skinner suggests: “It is possible that in this last statement, Jesus was playing off a well-known rabbinic chastisement that used the image of a tree. He [Rabbi Eliezer ben Azarya] used to say: ‘One whose wisdom exceeds his deeds, to what is he like? To a tree that has many branches and few roots, so that when the wind comes, it plucks it up and turns it over.’”
Comparison: There are no changes from the Matthean text.