False prophets look harmless from the outside, so Jesus gives His hearers a way to tell the difference. He asks a question every farmer could answer: nobody goes to a thorn bush for grapes. What a plant produces shows what it is. Barnes says, "The Saviour gives the proper test of their character," and adds, "The profession may be fair; but the 'conduct' - the fruit - is to determine the nature of the principles." Matthew Henry looks especially at their teaching: "They may be known by the drift and effects of their doctrines." Either way, the question trains the hearers to look past the sheep's clothing to what a teacher's life and message actually produce.

The answer is plainly no, and Jesus spells it out: "Even so every good tree bringeth forth good fruit; but a corrupt tree bringeth forth evil fruit" (Matthew 7:17). "A good tree cannot bring forth evil fruit, neither can a corrupt tree bring forth good fruit" (Matthew 7:18). He warns that the fruitless tree "is hewn down, and cast into the fire," and repeats His opening words: "Wherefore by their fruits ye shall know them" (Matthew 7:19-20).

The question begins in Greek with mēti, which Thayer describes as "an interrogative expecting a negative answer." The form of the question signals that the answer is no. The thorns (akantha) and thistles (tribolos) stand in contrast to grapes and figs, which Gill says "were common fruit" in Judea. He says plainly that "grapes do not grow upon, and are not to be gathered from thorns, and bramble bushes."

Luke records a similar saying with the pairs reversed: "For of thorns men do not gather figs, nor of a bramble bush gather they grapes" (Luke 6:44).

This question sits in the closing warnings of the Sermon on the Mount, where Jesus turns from teaching to testing. He has spoken of the narrow way (Matthew 7:13-14). Now He warns that not everyone who points to that way is trustworthy. In the very next verses, He goes further: even those who say "Lord, Lord" and do mighty works may not be His (Matthew 7:21-23). Fruit, not words or appearance, shows what is real.

Jesus uses the same picture later with the Pharisees: "the tree is known by his fruit," and "out of the abundance of the heart the mouth speaketh" (Matthew 12:33-34). What is inside will come out in time.