What can we do when we know someone who’s a false teacher? What does the Bible say about false teachers?
The Bible clearly indicates that there will be false teachers in the latter days. In fact, they were even around in the days of the apostles, but false teachers are not something new. Jeremiah the Prophet wrote that “Both prophet and priest are ungodly; even in my house I have found their evil, declares the Lord” (Jer 23:11), and even with “the prophets of Jerusalem I have seen a horrible thing: they commit adultery and walk in lies; they strengthen the hands of evildoers, so that no one turns from his evil; all of them have become like Sodom to me” (Jer 23:14), so just as it is in our day, it is “from the prophets of Jerusalem ungodliness has gone out into all the land” (Jer 23:15b). That’s why the Apostle John  tells us to “not believe every spirit, but test the spirits to see whether they are from God, for many false prophets have gone out into the world” (1st John 4:1). Ezekiel records God’s rebuke of the false prophets in ancient Israel, writing, “Son of man, prophesy against the prophets of Israel, who are prophesying, and say to those who prophesy from their own hearts: ‘Hear the word of the Lord!’ Thus says the Lord God, Woe to the foolish prophets who follow their own spirit, and have seen nothing” (Ezk 13:2-3).
The Apostle Paul, in mentoring and training Timothy, warned him that “If anyone teaches a different doctrine and does not agree with the sound words of our Lord Jesus Christ and the teaching that accords with godliness” (1st Tim 6:3), and are “people who are depraved in mind and deprived of the truth, imagining that godliness is a means of gain” (1st Tim 6:5). When Jesus was speaking to His disciples, He warned them that as the day of His return neared, “false christs and false prophets will arise and perform great signs and wonders, so as to lead astray, if possible, even the elect” (Matt 24:24). Paul also wrote that “the time is coming when people will not endure sound teaching, but having itching ears they will accumulate for themselves teachers to suit their own passions, and will turn away from listening to the truth and wander off into myths” (2nd Tim 4:3-4). Clearly, we know there will be false prophets and false teachers or pastors, so what do you do when you meet one? What do you say? What are the best ways to approach a false teacher?
I believe the first step in approaching a false teacher is to pray about it. Ask God to give you a love for that person and a genuine care for their soul, but also pray for the boldness to speak the truth. Pray that God brings to your mind some helpful Scriptures that can show this person where they’re wrong, and do this while praying for the Spirit of God to work in that person’s heart. You should pray for God to open their heart and reveal the fact that they’re teaching and believing error. It’s not about winning an argument or debate to prove they’re wrong. It’s not that we are better than they but that we want to help them see the error of their way. Focus on the fact that it’s not that our way or our teaching is better, but it’s “Thus says the Lord.” I think it’s even helpful to point out when you’ve been wrong, even admitting your own theological mistakes (which I believe we’ve all had). In no way should we ever come across as more biblically literate or superior in theological knowledge. Knowledge can puff up (1st Cor 8:1). We should correct a brother or sister in love, with meekness and humility, and remember that we are not the final authority…God’s Word is. Above all things, do it in love and soak it in prayer.
Parents couldn’t correct their children if they didn’t love their children, and in the same way, God equates discipline with love since “the Lord disciplines the one he loves, and chastises every son whom he receives” (Heb 12:6). Love must be the underlying motivation when confronting a false teacher. If we do it in a self-righteous attitude, we’re not doing it in love, and we can do more harm than good, causing them to dig in their heels even more, but just as the Lord disciplines us in His love, we must correct those who are teaching heresy or error in love. The Bible teaches, “It is for discipline that you have to endure. God is treating you as sons. For what son is there whom his father does not discipline” (Heb 12:7), since “If you are left without discipline, in which all have participated, then you are illegitimate children and not sons” (Heb 12:8). Think about this; maybe someone you think is a false teacher has simply misinterpreted some Scriptures or maybe they were taught by a false teacher. They might not even know that what they’re teaching is wrong, but it’s every believer’s duty to correct a fellow believer  if they are teaching error, but it must be motivated out of a love for your brother or sister.
One of the best ways to confront a false teacher is to use the Word of God. For instance, if someone is teaching that God still commands certain holy days to be observed, you could lovingly point out that “all who rely on works of the law are under a curse; for it is written, “Cursed be everyone who does not abide by all things written in the Book of the Law, and do them” (Gal 3:10). Tell them, by using the Word of God, that “it is evident that no one is justified before God by the law, for ‘The righteous shall live by faith’” (Gal 3:11). If it’s about trying to keep God’s law, then remind them that “Christ redeemed us from the curse of the law by becoming a curse for us—for it is written, ‘Cursed is everyone who is hanged on a tree’” (Gal 3:13), so it may be that they misunderstood what they were taught or don’t understand what grace is. The point is, we know that the law doesn’t save anyone, but that doesn’t mean we forget the law just because we’re redeemed from the curse of the law. Use the Word of God and perhaps the Spirit of God will open their eyes to the truth.
I have known a couple of godly men who allowed pride to take center stage in their heart, and after that, they began teaching things that the Bible clearly does not teach, so they began to drift away from sound teaching…slowly, further and further away from the truth, eventually creeping into error. Perhaps it happened so slowly that they didn’t even notice it, so approach a false teacher in love, and in a way that upholds the Word of God, but do nothing without prayer. False teachers are the reason that Jude “found it necessary to write appealing to you to contend for the faith that was once for all delivered to the saints. For certain people have crept in unnoticed who long ago were designated for this condemnation, ungodly people, who pervert the grace of our God into sensuality and deny our only Master and Lord, Jesus Christ” (Jude 1:3-4). You may also have to contend for the faith, but that doesn’t mean you have to be contentious.
Read this related article: How to Identify a False Prophet or Teacher 
Resource – Scripture quotations are from The Holy Bible, English Standard Version® (ESV®), Crossway Bibles. (2007). ESV: Study Bible : English standard version. Wheaton, Ill: Crossway Bibles. Used by permission. All rights reserved.