7:15-20 Nothing so much prevents men from entering the strait gate, and becoming true followers of Christ, as the carnal, soothing, flattering doctrines of those who oppose the truth. They may be known by the drift and effects of their doctrines. Some part of their temper and conduct is contrary to the mind of Christ. Those opinions come not from God that lead to sin.

16. Ye shall know them by their fruits—not their doctrines—as many of the elder interpreters and some later ones explain it—for that corresponds to the tree itself; but the practical effect of their teaching, which is the proper fruit of the tree.

Do men gather grapes of thorns—any kind of prickly plant.

or figs of thistles?—a three-pronged variety. The general sense is obvious—Every tree bears its own fruit.