Explain “Good” and “Bad” Fruit

Please explain Jesus' teaching on good and bad fruit from Matthew 7:18-20? How does this relate to false teachers?

The verses you asked us to explain are part of the following larger passage:

Matt. 7:15 “Beware of the false prophets, who come to you in sheep’s clothing, but inwardly are ravenous wolves.
Matt. 7:16 “You will know them by their fruits. Grapes are not gathered from thorn bushes nor figs from thistles, are they?
Matt. 7:17 “So every good tree bears good fruit, but the bad tree bears bad fruit.
Matt. 7:18 “A good tree cannot produce bad fruit, nor can a bad tree produce good fruit.
Matt. 7:19 “Every tree that does not bear good fruit is cut down and thrown into the fire.
Matt. 7:20 “So then, you will know them by their fruits.
Matt. 7:21 “Not everyone who says to Me, ‘Lord, Lord,’ will enter the kingdom of heaven, but he who does the will of My Father who is in heaven will enter.
Matt. 7:22 “Many will say to Me on that day, ‘Lord, Lord, did we not prophesy in Your name, and in Your name cast out demons, and in Your name perform many miracles?’
Matt. 7:23 “And then I will declare to them, ‘I never knew you; depart from Me, You who practice lawlessness.’

Notice that Jesus is speaking about false prophets. False prophets (or teachers) are those who claim to know the Lord and to follow Him but actually are not truly His disciples. They are false in every respect: they do not know the Lord, they do not know His word, nor do they following His commandments. Therefore, they are “wolves” masquerading as “sheep.”

Christ tells His true followers to avoid falling victim to these false teachers by evaluating the “fruit” in their lives. Fruit refers to the product of their ministry. Just as a tree’s output is the fruit it yields after a season, so is a teacher’s production measure by the lasting effect (i.e., fruit) they yield in the lives of their students.

Jesus teaches the principle that true teachers of God’s word will naturally and always produce good outcomes in the lives of their students because these teachers are sharing the true word of God. The word of God itself produces the fruit in the lives of the hearers, and since the teacher is sharing the word in an honest way, that teacher will naturally yield good outcomes.

On the other hand, false teachers or prophets do not teach the word of God. Instead, they teach man-made conclusion in place of the word of God, as Jesus said a few verses earlier in this chapter:

Mark 7:7 ‘But in vain do they worship Me,
Teaching as doctrines the precepts of men.’

Those who spread false teaching will always yield “bad fruit,” which refers to the absence of godliness in the lives their students. Those who listen to false teaching do not experience changed lives or develop greater love for and devotion to Christ Jesus and His word. They may experience a season of excitement or enthusiasm, but in the end, they cannot grow spiritually nor show the fruit of the Spirit in their lives since they lack the pure milk of the word of God, as Peter says:

1Pet. 2:2 like newborn babies, long for the pure milk of the word, so that by it you may grow in respect to salvation,

Simply put, false teaching is not from God and therefore it lacks the power of God’s word. So Jesus warns the believer to evaluate the product of a person’s teaching to know whether such a person is teaching truthfully or not. In v.17-18 Jesus says that those who teach God’s word cannot yield good fruit, since only God’s word has the power to produce that outcome. Likewise, those who teach God’s word honestly will always see good fruit, because the word of God will not go forth and return void, as Isaiah says:

Is. 55:10 “For as the rain and the snow come down from heaven,
   And do not return there without watering the earth
   And making it bear and sprout,
   And furnishing seed to the sower and bread to the eater;
Is. 55:11 So will My word be which goes forth from My mouth;
   It will not return to Me empty,
   Without accomplishing what I desire,
   And without succeeding in the matter for which I sent it.

Nevertheless, false teachers will portray themselves as pious examples to those they mislead. They are like the Pharisees who disingenuously called Jesus “Teacher” or “Rabbi” when in reality they despised him and were conspiring to kill HIm. Jesus was speaking of such hypocrites when He said in Matt 7:22-23 there will be those who called Him respectful names (e.g., “Lord") in the day He walked the earth who were not truly His disciples. He was speaking of false teachers, His enemies who merely portrayed themselves as friends for self-serving purposes.

Jesus’ point was to warn the faithful against coming under the influence of false teachers merely because they invoke the name of Jesus or live pious lifestyles. Mormons are a classic example of such deceptive influence. Mormonism is a false church with false prophets teaching false doctrines, yet they invoke the name of Jesus (calling Him “Lord”) and they live pious lives in the public eye. Despite these outward displays, Mormons believe (and preach) a false Gospel, and therefore they will not enter the kingdom of Heaven.