The Lord warned His followers to "beware of false prophets." The term "prophets" technically only includes those who claim inspiration, though practically speaking it would include any teacher who claims to be delivering a message from deity (either directly via inspiration or indirectly via the Scriptures). Consequently, Jesus' warning should be considered as potentially applicable to any who claim to teach God's ways to men--myself included. A false teacher should be characterized as one whose teachings do not harmonize with God's word and as one who is unwilling to repent of his erroneous views when brought to his attention. Although there are many teachers with an outward demeanor of innocence and gentleness, Jesus says that some of them are ravenous wolves inwardly (cf. II Cor. 11:13-15). They will devour many souls, leading themselves and others down the pathway to destruction (II Pet. 2:1-3).
But, how can one distinguish between a sheep and a wolf in sheep's clothing, since their appearance is essentially the same? The answer: We must examine their "fruit"! That is, we must examine their actions and their teachings carefully and determine if such are in harmony with God's word or not. We must make judgments of this sort all the time. Even at this very moment, you should be examining my words and judging whether or not I am speaking the truth (cf. Acts 17:11).
It is entirely possible to have a good tree and a bad tree that look nearly identical in trunk, limbs, and leaves, yet they differ in the quality of fruit they produce. This is why we must be "fruit inspectors." And, when one inspects "fruit," he must use God's standard for determining what is good (cf. Gal. 5:22,23). Eventually, the hypocrisy of all false teachers will be exposed by those who observe them carefully. Such must be true because, as the Lord declared - "A bad tree bears bad fruit."
It would be unwise to interpret Jesus' statement in Matthew 7:18 in an absolute way. Surely there are good trees physically that may occasionally produce a bad piece of fruit. Likewise, it would seem reasonable to suggest that a bad tree might occasionally produce a good piece of fruit. Jesus' point should be viewed as a general truth. Good trees are the ones that generally produce good fruit. If a tree is producing a quantity of bad fruit, then that tree itself cannot be good. The same is true with people.
The Scriptures teach elsewhere that the duty of man is to fear God and obey Him (Eccl. 12:13). Fulfilling this duty is synonymous with bearing good fruit. If one isn't fulfilling the duty for which He was created, God will eventually throw him into the eternal fire.
Friends, God calls us to be fruit inspectors. While we have no right to sentence and condemn others (cf. Matt. 7:1), we are to discern or judge whether or not a person is a false teacher. We should not blindly accept or support any spiritual idea or religious person. We must carefully test the fruit and hold fast to that which is good (I Thess. 5:21)! And, may we realize that others are examining the fruit we produce! If you're a "good tree", keep producing for the Lord! If you're a "bad tree", repent while you still can--for the axe of God's judgment and the fire of His wrath are approaching!