Matthew 7:15-20
15 Beware of false prophets, which come to you in sheep's clothing, but inwardly they are ravening wolves.
16 Ye shall know them by their fruits. Do men gather grapes of thorns, or figs of thistles?
17 Even so every good tree bringeth forth good fruit; but a corrupt tree bringeth forth evil fruit.
18 A good tree cannot bring forth evil fruit, neither can a corrupt tree bring forth good fruit.
19 Every tree that bringeth not forth good fruit is hewn down, and cast into the fire.
20 Wherefore by their fruits ye shall know them.
The Grace New Testament Commentary - Matthew 7:15-20
7:15-20 One reason so few are entering the kingdom is because of the many influential "spokesmen for God" who are promoting the wide gate and broad path. These teachers are not readily perceived as "false prophets," for they are wolves "in sheep's clothing." Their predatory nature is hidden by an innocent appearance. They cloak their false teaching with what seems to be good works. But as Jesus said, "'they say and do not do'" (23:3). Pharisaical righteousness will not get anyone into the kingdom.
Jesus teaches that a plant or tree is known "by [its] fruit." Their ingenious means of reducing the demands of the Law to something people can keep for salvation betray who they really are. The false teacher is likened to "a bad tree" that naturally produces "bad fruit" whose end is destruction by fire.
These false teachers are detected "by their fruits." The fruit of the false teacher is not his works, for these appear to be good. Rather the false fruit is their doctrine (cf. 12:33-35). In the case of the Pharisees, it was legalism - that one could merit acceptance with God by his deeds. Testing doctrine is the means by which the seeker can discern a teacher's true nature. To be able to test someone's doctrine, one must be well versed in the Word of God.
Today many false ideas are being taught. Paul wrote of false apostles who offered a counterfeit Christ, a counterfeit Spirit, and a counterfeit gospel (2 Cor 11:4, 13-15). Satan loves to enlist religious teachers who obscure the gospel of grace.