What do you say to someone who is listening to a false teacher?
What is a Heretic?
If you looked at a dictionary’s definition of a heretic and the Bible’s description of one, you would find some very strong similarities. For example, the dictionary says a heretic is a professed believer but they maintain religious opinions contrary to those accepted by their church or they reject doctrines prescribed by that church, but it could be anyone who does not conform to an established attitude, doctrine, or principle. The word heretic originated from the Greek word “hairetikós,” which means, “to be able to choose,” or “that which may be taken,” so it reminds me of a smorgasbord approach to religion. They pick a little of this and choose a little of that, but they leave much off their plate. Jesus said it is important to worship God in spirit and in truth (John 4:24), and a half-truth is no truth at all. In biblical terms, a heretic is someone who believes and may be teaching doctrine contrary to that which the Bible contains (i.e. Eph 2:8-9). A heretic usually refers to someone who has departed from what they used to believe in and now teaching what is contrary to what they used to believe, and heretics can be in places other than churches, but for our concerns, we want to know what a heretic is so that we can spot one, and if we can spot one, we can avoid one, and we also want to make sure we’re not one! 
There are many synonyms for the word heretic, and they seem perfectly fitting. Other names for heretics are apostates, backsliders, dissenters, skeptics, or freethinkers. If they are basing their beliefs on “free thinking,” then you can be almost certain it’s not based upon truth…and if it’s not truth, then its heresy. Satan has his own ministers, and in appearance, they look just as saintly as can be. Obviously, they won’t look like Satan’s ministers, but rather will appear as angels of light (2 Cor 11:4). I don’t think anyone falls into these false teachings, thinking, “Okay, now I’m going to be deceived.” It happens gradually, almost without notice. Over time, certain biblical principles and doctrines are compromised. One man said we don’t need to mention hell or repentance but just tell them, “God loves you,” but the problem with that is that’s only half the truth. Jesus said the gospel of the kingdom of God consists of repentance and faith (Mark 1:14-15), and although God grants repentance (2 Tim 2:25), God commands all men everywhere to repent (Acts 17:30), so it is not enough to say “God loves you,” but rather God requires us to make disciples of all nations, baptizing them, and teaching them the very same things Jesus taught His disciples (Matt 28:18-20). Sinners must know that God’s wrath is being stored up against them (Rom 2:5), so they must run to the cross for mercy. If they never know about sin and the wrath of God, they will never see the relevance of God’s mercy. God’s wrath makes God’s mercy relevant. “God loves you” doesn’t seem to make people seek forgiveness, but rather they only end up with a worthless “warm fuzzy.” That however, is not conversion. The gospel without repentance and faith is heresy, and that is spoken by heretics, and sadly, sometimes heretics are found in the churches.
Differences not Heresy
It breaks my heart to have to say that I’ve seen Christians call each other heretics over things that are non-essential. One man called his brother a heretic because he didn’t believe in tongues. The other said he was a heretic because he believed in the Mid-Trib rapture and not a Post-Trib view, but these differences in belief are not grounds for calling someone a heretic. It’s wrong to use that name against someone who believes differently than they do. Certain things are not essential to salvation, so to have differences of opinions or to hold to different views doesn’t mean everyone else who doesn’t agree with us is a heretic. If that person says that Jesus was not God, then that’s heresy, and heretics speak heresies, and they must be silenced, at least in the church, so unless someone questions the divinity of Christ, the sinless-ness of Jesus, or His being our Lord and Savior, we can’t call someone a heretic for things that are not critical to our salvation. One man claimed that if you didn’t speak in tongues, you weren’t saved, but to me, that comes dangerously close to heresy. It’s a work-based theology, and that doesn’t fly with the gospel of Jesus Christ (Eph 2:8-9), but your friend is not a heretic just because he believes pets don’t go to heaven. Heresy is what undermines the gospel, not areas where the Bible is silent.
I don’t think I need to convince you that there are many false prophets out there today, and that shouldn’t surprise us because Jesus said one of the signs leading up to His second coming would be that “many false prophets will arise and lead many astray” (Matt 24:11). Notice Jesus said “many” and not a few. And they will even come in His name but will “deceive many” (Matt 24:5), again, “many” will be deceived and not just a few, so we are warned, again by Jesus, to “Beware of false prophets, who come to you in sheep’s clothing but inwardly are ravenous wolves” (Matt 7:13), so there will be many, not a few. We must contend for the faith once delivered (Jude 1:3) because many pretend to offer you a “great gospel” message; a real-good, feel-good message, but “there is another one, but there are some who trouble you and want to distort the gospel of Christ” (Gal 1:7). Paul told the church at Corinth that “if someone comes and proclaims another Jesus than the one we proclaimed, or if you receive a different spirit from the one you received, or if you accept a different gospel from the one you accepted, you put up with it readily enough” (2 Cor 11:4), so “even if we or an angel from heaven should preach to you a gospel contrary to the one we preached to you, let him be accursed” (Gal 1:8). To put an exclamation mark on it, Paul says, “again: If anyone is preaching to you a gospel contrary to the one you received, let him be accursed” (Gal 1:9). Any other gospel is heresy and anyone who teaches it is a heretic, plain and simple.
There are heretics in the churches today, but perhaps many of these churches are not Jesus’ churches after all, but perhaps Satan’s churches and with Satan’s ministers. And on TV, they certainly look like angels of light, but in their teachings lie darkness and lies, from the father of lies (John 8:44). There is heresy in teaching that Jesus was not fully Man and fully God; that Jesus is not God and has always been God and fully divine; that Jesus did not live a sinless life of perfection, making a satisfactory sacrifice to satisfy the wrath of God; and that Jesus was not raised on the third day and now sits at the right hand of the Father, directing His church. Any other doctrine contrary to these teachings is to “distort the gospel of Christ” (Gal 1:7b), and teach another gospel, “not that there is another” (Gal 1:7a).
Here is some related reading for you: 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.