A good tree cannot bear bad fruit, and a bad tree cannot bear good fruit.
We have here a caution against false prophets, to take heed that we be not deceived and imposed upon by them. Prophets are properly such as foretel things to come; there are some mentioned in the Old Testament, who pretended to that without warrant, and the event disproved their pretensions, as Zedekiah, Kg1 22:11, and another Zedekiah, Jer 29:21.
Commenting on Matthew 7:15-20
Wherefore by their fruits ye shall know them. This is the conclusion of the whole, and a repetition of what is before said, the more to fix the rule of judgment upon their minds, and engage them to try men by their doctrines, and their doctrines by the standard of the Scriptures, and not believe every spirit; for with some care and diligence such persons...
CONCLUSION AND EFFECT OF THE SERMON ON THE MOUNT. (Mat. 7:13-29) Enter ye in at the strait gate--as if hardly wide enough to admit one at all. This expresses the difficulty of the first right step in religion, involving, as it does, a triumph over all our natural inclinations.
Commenting on Matthew 7:13-29