Luke 6:43

For there is no good tree that bringeth forth corrupt fruit; nor again a corrupt tree that bringeth forth good fruit. For each tree is known by its own fruit. For of thorns men do not gather figs, nor of a bramble bush gather they grapes.

In Matthew (Matthew 7:17-20) this teaching was applied to the identification of false teachers. No corrupt teacher can produce desirable results. It would be as logical to expect a bucket of figs to grow on a thorn bush as to expect holy and beneficial results to follow from a teacher who is not faithful to the word of God. Social excellence, eloquent speech, personable appearance, fashionable attire, and charming demeanor on the part of a teacher are not sufficient reasons for following one who does not know, or will not proclaim, the true word of God.