Even so, every good tree bears good fruit, but a bad tree bears bad fruit. A good tree cannot bear bad fruit, nor can a bad tree bear good fruit. Matthew 7:17-18

Clearly, the Lord was not saying that believers produce only good fruit and unbelievers produce only bad fruit. We know that those who believe in Christ sin and that unbelievers sometimes do good things.

So, what did the Lord mean?

Sadly, most commentaries say that the people to be examined are false professors and that their fruit are their bad works. See, for example, this Gotquestions.org article. The article concludes, “We can identify those whose hearts have been redeemed by the fruit we see in their lives.”

Likewise, Bibleref.com (see here) writes concerning Matt 7:17,

Christ clarifies that point even further, here. Healthy trees bear good fruit, sick trees bear bad fruit. That law of nature is true of people, as well as trees. “Healthy” prophets and teachers demonstrate through their lives and character that they are authentic messengers of God (emphasis added).

The Bible paraphrase The Message directly contradicts the point the Lord was making (see here):

15-20 “Be wary of false preachers who smile a lot, dripping with practiced sincerity. Chances are they are out to rip you off some way or other. Don’t be impressed with charisma; look for character. Who preachers are is the main thing, not what they say. A genuine leader will never exploit your emotions or your pocketbook. These diseased trees with their bad apples are going to be chopped down and burned (emphasis added).

The context is about false prophets (v 15). The bad fruit is false teaching, not bad behavior. After all, the false prophets look like sheep.

Note this helpful comment from PreceptAustin.org concerning Matt 7:17-20 (see here under bad fruit):

…false prophets and false teachers know what people want to hear and so they proclaim a message that appeals to sensual desires (see notes beginning 2 Peter 2, especially 2Pe 2:2-note). These spiritual charlatans don’t wear “warning labels”, but are servants of Satan who “disguise themselves as servants of righteousness” (2Co 11:15). They are masterful spiritual chameleons who make stupendous claims, but in due time their followers usually end up paying a high price, which can even be eternal destruction if they never hear the truth, are granted repentance, come to their senses and escape from the snare of the devil’s servants, having been held captive by them to do their will (2Ti 2:24, 25, 26-notes). Even believers can be deceived by false prophets and false teachers. That is why God’s Word repeatedly exhorts us to study the Scriptures (1Pe 2:2-note), test what we hear (1Jn 4:1+, Acts 17:11-note), and grow in the faith (2Pe 1:5, 6, 7, 8, 9 – see notes 2Pe 1:5, 6-7, 8-9) (emphasis added).

The Lord was not saying that false teachers never say anything that is true. He was saying that they teach falsely on one or more fundamental doctrines. That is, a false teacher is evident in that he denies a truth such as the deity of Christ, justification by faith alone, the inerrancy of Scripture, eternal security, the Trinity, etc. “You will know them by their fruits” (Matt 7:16a).

Neither is Jesus saying that good teachers never say anything false. However, good teachers do not teach error on any fundamental doctrine. You know true teachers by what they teach.ⁱ

A Bible teacher shows whether he a true or false teacher by what he says, not by what he does.

The Lordship idea that the people in question in these verses can be determined by their fruits is not completely wrong. The Lordship interpretation simply misses two points. First, the people in question are false prophets, not unbelievers. Second, their fruits are their teachings, not their works. But it is true that the Lord is saying that we are to examine something about those who teach God’s Word. We are to examine what they teach.

True teachers teach the truth on fundamental doctrines. False teachers teach error on one or more fundamental doctrines.

Keep grace in focus and you will be able to tell whether a Bible teacher is accurate or inaccurate.

ⁱ There is ambiguity in the Lord’s teaching here. His point is that you know Bible teachers by their teachings. If one’s teaching is generally Biblical, then they are a good teacher. If their teaching is generally unbiblical, then they are a false teacher. But the Lord does not give a precise way for determining which Bible teachers are bad trees and which are good trees. His point is that we should look at their teachings, not their sheep’s clothing.