15"Beware of false prophets, who come to you in sheep's clothing, but inwardly they are ravenous wolves. 16You will know them by their fruits. Do men gather grapes from thornbushes or figs from thistles? 17Even so, every good tree bears good fruit, but a bad tree bears bad fruit. 18A good tree cannot bear bad fruit, nor can a bad tree bear good fruit. 19Every tree that does not bear good fruit is cut down and thrown into the fire. 20Therefore by their fruits you will know them." - Matthew 7:15-20
This passage is often used to argue that a person's works will be proof of his or her salvation. It assumes that "fruits" refers to visible conduct that can be quantified in such a way that others can pass judgment on that person's salvation ("you will know them"). According to this interpretation, bad behavior proves a person is not saved; good behavior proves a person is saved. Is that what this passage teaches?
It should be noted first that Jesus is not addressing believers or professing believers in general, but false prophets and how to recognize them. To be exact, the test is not for judging the reality of another's salvation, but for judging whether a prophet is from God or not.
Context clarifies the focus of the passage. These statements are from the Sermon on the Mount where Jesus Christ is explaining the highest standards of righteousness that characterize the kingdom. It is a righteousness that exceeds that of the Scribes and Pharisees (Matt. 5:20). The Scribes and Pharisses were highly scrupulous in their behavior, so it seems unlikely that Jesus' reference to "fruits" would focus on conduct. Likewise, the following passage in 7:21-23 mentions those who do great things in Jesus' name, but Jesus ignores the significance of those professors' works.
The prophets first appear deceptively as true believers ("in sheep's clothing"). They are indiscernible from believers in what can be seen. They are evidently clothed in a façade of Christian behavior which proves to be an inadequate basis of judgment. It is only what is unseen that later proves them false prophets.
The test that Jesus gives is not for the existence of fruit, but for the quality of fruit (v. 17). The false prophet may have fruits, but given time to ripen, they prove "bad" (v. 16). Likewise, a tree cannot be judged good or bad from its outer appearance, but from the fruit it produces (vv. 17-18). The true test of a prophet is whether his fruits are good or bad. But what does "fruits" refer to?
If "fruits" refers only to works, this creates a couple problems. First, many false religions produce teachers and adherents with good moral conduct and good works. Second, there would be a conflict with the following verses, 21-23, where the professors have good works, but the Lord says He never knew them.
"Fruits" must certainly refer to more than works; words must be in view. In Matthew 12:33-37 there is a similar discussion about fruits that shows they are one's words:
33"Either make the tree good and its fruit good, or else make the tree bad and its fruit bad; for a tree is known by its fruit. 34Brood of vipers! How can you, being evil, speak good things? For out of the abundance of the heart the mouth speaks. 35A good man out of the good treasure of his heart brings forth good things, and an evil man out of the evil treasure brings forth evil things. 36But I say to you that for every idle word men may speak, they will give account of it in the day of judgment. 37For by your words you will be justified, and by your words you will be condemned."
This is how a person proves his true nature. Given time, what is beneath the deceptive façade is exposed in his words. Word express one's beliefs, thus they are the basis for vindication or condemnation.
The Mosaic Law also prescribed the test of a false prophet. In Deuteronomy 13:1-3 the Israelites are told to ignore any miraculous works of a so-called prophet and judge him only by his words. Likewise, in Deuteronomy 18:18-22 the validity of a prophet of God ultimately depends on his words, whether they are true or false, fulfilled or not.
Words of a teacher or any person will eventually betray his or her beliefs. Outward conduct can be deceiving and is not a reliable judge of the reality of one's faith. A person can only be judged by what he or she says when compared to the truth of the Bible. The Word of God is the final judge of a teacher's credibility or a person's salvation. Don't be fooled by someone's works; they are not a reliable basis for judging (See GraceNotes no. 28, "Can Good Works Prove Salvation?"). If we are saved by grace through faith, then one's verbal testimony should affirm that truth according to God's Word. We would hope that one's conduct is consistent with that profession.