Matteo 7:18

A good tree cannot bring forth evil fruit, neither can a corrupt tree bring forth good fruit.

A significant point: Not only may the disciples of Christ distinguish these false teachers for themselves, but the Lord expects them to know them thoroughly, to understand them by making a study of their methods and their way of life. Christians are able, they have the sacred duty, to try the spirits, to examine and test the doctrine which is offered to them. They have an infallible rule, the teaching of Christ, the Word of Truth.

According to this criterion, this standard, they should judge not only the doctrine, but also the works of the false teachers, which are here called their fruits. Men never think of collecting grapes from thorns or figs from thistles. They are not deceived by false resemblances, just as the botanist will tell at a glance the poisonous variety of berry or mushroom from the good. But even where so much botanical knowledge is not found, the good, the sound, healthy tree is readily distinguished from the unhealthy, the degenerate tree, standing in bad soil, or no longer fruitful on account of age.

All these trees and plants bear in accordance with their nature, this test never fails. "As we perfectly know that a good tree will not produce bad fruit, and the bad tree will not, cannot produce good fruit, so we know that the profession of godliness, while the life is ungodly, is imposture, hypocrisy, and deceit."