We assume "know the tree by the fruit" to mean, for example, if one bears no or bad fruit, he was never saved, to begin with, which is akin to the fox assuming in the Aesop Fables - "the grape must be sour!" - an example of making a subjective, biased conclusion.
The fox and grape: (Aesop Fables)
A Fox one day spied a beautiful bunch of ripe grapes hanging from a vine trained along the branches of a tree. The grapes seemed ready to burst with juice, and the Fox’s mouth watered as he gazed longingly at them. The bunch hung from a high branch, and the Fox had to jump for it. After several failed attempts, the fox gave up and walked away saying, "the grapes were too sour."
The fruit is the positive ID of the kind of tree. For, the tree cannot determine what to bear, nor can it change by itself according to the circumstances against the laws of nature. But man, created in the Image of God, can and do all the things trees cannot do. We can determine our behavior and flips back and forth at times.
In the context, the storyline of Jesus crosses over from the natural realm of "tree-fruit" to man's spiritual reality. Also, Jesus mentions "cutting down and throwing into the fire" (on the last day). It is a Biblical term for the final consequences of men of "bad fruit," but not of the bad fruit-bearing tree/branches.
- How the "cutting - line" affect the understanding of the unprofitable servant (parables of Talents & Mina), the five foolish virgins, and the workers of lawlessness (Mt 7:23)?
Lk 6: 43-45
“For no good tree bears bad fruit, nor again does a bad tree bear good fruit, for each tree is known by its own fruit. For figs are not gathered from thornbushes, nor are grapes picked from a bramble bush. The good person out of the good treasure of his heart produces good, and the evil person out of his evil treasure produces evil, for out of the abundance of the heart his mouth speaks.
Mt 7:15-20 (ref. 12:33b)
“Beware of false prophets, who come to you in sheep’s clothing but inwardly are ravenous wolves. 16 You will recognize them by their fruits. Are grapes gathered from thornbushes, or figs from thistles? 17 So, every healthy tree bears good fruit, but the diseased tree bears bad fruit. 18 A healthy tree cannot bear bad fruit, nor can a diseased tree bear good fruit. 19 Every tree that does not bear good fruit is cut down and thrown into the fire. 20 Thus you will recognize them by their fruits."