By Their Fruits
15 “Beware of false prophets, who come to you in sheep’s clothing, but inwardly are ravening wolves. 16 By their fruits you will know them. Do you gather grapes from thorns or figs from thistles? 17 Even so, every good tree produces good fruit, but the corrupt tree produces evil fruit. 18 A good tree can’t produce evil fruit, neither can a corrupt tree produce good fruit. 19 Every tree that doesn’t grow good fruit is cut down and thrown into the fire. 20 Therefore by their fruits you will know them.
The second contrast: teachers dressed as sheep, and the one test that cannot be faked because it takes a season.
“Beware of false prophets, who come to you in sheep’s clothing, but inwardly are ravening wolves.
The warning is about teachers rather than unbelievers, and the danger is stated as disguise. These are not people identifiable by obvious wickedness; they arrive inside the flock, dressed as members of it, and the sermon assumes they will be persuasive. The first thing said about them is that vigilance is required, which implies they can be recognized — but not on sight.
By their fruits you will know them. Do you gather grapes from thorns or figs from thistles?
The test is deliberately slow. Fruit takes a season, cannot be manufactured on demand, and appears whether or not the tree intends it — which makes it the one indicator a disguise cannot produce. Charisma, doctrine recited correctly, and results of the kind listed a few verses later are all available to a wolf; a life is not.
What counts as fruit is not defined here, and the sermon that has just run for two chapters is the definition. Mercy, purity, peacemaking, the treasure relocated, the enemy prayed for, the plain yes — these are what a good tree grows.
A good tree can’t produce evil fruit, neither can a corrupt tree produce good fruit.
The claim is stronger than that bad people do bad things: it is that the fruit reveals the nature of the tree, reliably, in both directions. That is why the test works, and it is also the sermon's answer to the question of how the righteousness that exceeds Matthew 5:20 is ever produced. Trees are not exhorted into fruitfulness. What kind of tree it is decides the matter, and the beatitudes said at the outset who receives the kingdom.
Every tree that doesn’t grow good fruit is cut down and thrown into the fire.
The sentence is John the Baptizer's, word for word Matthew 3:10, which is the second time this Gospel has put the forerunner's preaching in Jesus' mouth Matthew 4:17. The repetition at the end of the sermon is a reminder that the ax was described as already lying at the root, and that the discourse which began by blessing the poor in spirit has not softened the warning it started from.
settled What this mark means