7:15 Jesus here sounded a warning that the Old Testament prophets also gave about false prophets (cf. Deut. 13; 18; Jer. 6:13-15; 8:8-12; Ezek. 13; 22:27; Zeph 3:4). He did not explain exactly what they would teach, only that they would deceptively misrepresent divine revelation. This covers a wide spectrum of false teachers. Their motive was self-serving, and the end of their victims would be destruction. These characteristics are implicit in Jesus' description of them. The scribes and Pharisees manned a narrow gate, but it was not the gate that led to the narrow way leading to life.
7:16-20 Fruit in the natural world, as well as metaphorically, represents what the plant or person produces. It is what other people see that leads them to conclude something about the nature and identity of what bears the fruit. Fruit is the best indicator of this nature. In false teachers, fruit represents their doctrines and deeds. Jesus said His disciples would be able to recognize false prophets by their fruit, their teachings and their actions. Sometimes the true character of a person remains hidden for some time. People regard their good works as an indication of righteous character. However eventually the true nature of the person becomes apparent, and it becomes clear that one's apparently good fruit was rotten.
Prophets true to God's Word would produce righteous conduct, but false prophets who disregarded God's Word would produce unrighteous conduct (v. 17).
A poisonous plant will yield poisonous fruit. It cannot produce healthful fruit. Likewise a good tree, such as an apple tree, bears good nutritious fruit (v. 18). The bad fruit may look good, but it is bad nonetheless (v. 16). A false prophet can only produce bad works even though his works may appear good superficially or temporarily.
Some interpreters of this passage take Jesus' teaching farther than He went with it. They say it is impossible for a genuine believer to do bad works. This cannot be true in view of the hundreds of commands, exhortations, and warnings that Jesus and the apostles gave to believers in the New Testament. It is possible for a believer to do bad works (e.g., 16:23; Tit. 2:11-13; 3:8; 1 John 1:9). That they will not is the teaching of sinless perfection. Other interpreters say that some bad works are inevitable for the believer, but bad works will not characterize the life of a true believer. This quickly turns into a question of how many bad works, which the New Testament does not answer. Rather the New Testament writers present some people who have departed from God's will for a long time as believers (e.g., 1 Tim. 1:20; 2 Tim. 2:17-18). The point Jesus was making in verse 18 was simply that false prophets do what is bad, and people who follow God faithfully do what is good. He already told His disciples not to judge one another (vv. 1-5). We should not judge whether a person is saved or lost by their works as the following alternatives clarify further.
The end of every tree that does not bear good fruit is the fire (v. 19). Likewise the false prophet who does bad works, even though they look good, suffers destructive judgment (cf. 3:10).
The words and works of a prophet eventually reveal his true character just as surely as the fruit of a tree reveals its identity (v. 20). Of these two criteria, works are the more reliable guide.
Jesus was evidently dealing with typical false prophets in this section. He did not go into the case of a believer who deliberately distorts God's Word. Typically a false prophet rejects God's Word because he is an unbeliever. However even in the Old Testament there were a few true prophets who lied about God's Word (e.g., 1 Kings 13:18).
