How to Spot a False Teacher
While reasoned gentleness should be our default response to those with whom we disagree, we should nevertheless reserve strong words for false teachers to underscore the seriousness of false teaching and to protect those whom false teachers harm. When approaching someone then, we need to discern whether or not someone is a false teacher or not. And this is no easy task nor is "false teacher" a phrase we should use regularly. For by saying it, we mean that someone has doomed themselves to a life without forgiveness.
A basic thesis
Here is how you spot a false teacher: a false teacher confesses false doctrine and harms others yet knows how to hide him or herself well. By definition, a teacher instructs others. So a false teacher must be in a position to influence or persuade others of false doctrine. And he or she must do so in such a way as to harm others. Yet false teachers often masquerade as sheep in wolves clothing following their spiritual father who masquerades as an angel of light but has no light in himself. So it's often hard to spot a false teacher because they are adept at disguise and hypocrisy. So the above definition contains four elements: (1) a position of teaching in which the teacher instructs others, (2) a wrong confession of doctrine, (3) a life that harms others, and (4) an ability to disguise oneself.
Four signs of a false teacher
Let's take that apart one-by-one to illustrate what I mean. First, false teachers instruct others. When the Bible talks about a false teacher, by definition these are ecclesial leaders who influence and persuade others to their theological tribe (2 Pet 2:1). Certainly, false teachers do not have to be elders or pastors (1 Tim 1:6). But they teach. Somehow and in some setting, they instruct others in a divergent faith. Second, false teachers teach false doctrine. False teachers deny the Master Jesus (Jude 4; 2 Pet 2:1), reject Jesus' divine origin (1 John 4:3), blaspheme the way of Christianity (2 Pet 2:2), follow their sensual desires (2 Pet 2:2), and prey on the weak and exploit others (2 Pet 2:3; 2 Tim 3:6; Rom 16:18). In short, their doctrine is not merely about intellectual assent but also about a way of life. Their way is greedy gain and their doctrine is to deny the Master who bought them. Third, false teachers harm others. As noted, false teachers prey on people and exploit them for their own devious ends. Yet the harm they commit goes beyond this. In Galatians, false teachers have the form of the Gospel but empty it of its power by adding works to the Gospel and by divesting the faith of its spiritual power (Gal 1:18; 3:3). In this sense, the false teachers are not sexually abusing widows, or exploiting the weak per se. The harm they are inflicting is to divest people of spiritual power—having begun by the Spirit, they encourage people to finish their faith by the flesh. They also add to the Gospel, diluting the Gospel so much that it fails to be saving (see Gal 1:6–7). Fourth, false teachers disguise themselves. So harm not only means sensual and exploitative harm against the body but also spiritual harm against the soul. And of course, this can often be hard to spot. After all, if Satan masquerades as an angel of light, will not his lieutenants also look like good, moral bible teachers?
Some cautions when considering calling someone a false teacher
A number of cautions are in order before using the above criteria to name a false teacher:
A Christian leader who falls morally is not a false teacher if he or she repents. If that leader does not repent, it's likely they were never apart of us or were a wolf in sheep's clothing.
A false teacher is not someone you disagree with. If someone teaches something contrary to you but in good faith, be quick to listen and slow to speak. Maybe you are wrong. Maybe the other person is. Some people believe in a 7-year tribulation, others do not. It's not a question of being a false teacher but of genuine differences.
A false teacher teaches contrary to the centre of Christianity: they deny Jesus (2 Pet 2:1; Jude 4), reject his divine origin (1 John 4:3), dilute the Gospel (Gal 1:6–7), and live by the flesh (Gal 3:6; cf. 1 John 2:16).
Our default position is always reasoned gentleness (1 Pet 3:15). But if someone has seared their conscience to the point of being unable to listen, then we should move into the warning stage. If this person persists in teaching falsely and harms others by this teaching and/or by his or her actions, then we should move into the "false teacher" stage.
A false teacher ends up being a religious hypocrite who knows the truth but denies it or has seared his or her own conscience to the point of being unable to know the truth ("always learning and never able to arrive at a knowledge of the truth"). So calling someone a "false teacher" means that they likely will never receive forgiveness of sins—that they are damned. So be very slow to call someone a false teacher. It means something deeply serious.
Who is a modern day false teacher?
Anyone whose life and doctrine harms others might be a false teacher. And yet this is not so easy to discern because false teachers hide, wearing the garments of sheep but inwardly being wolves. In light of the danger of false teachers, consider Paul's words to the Ephesian elders:
I know that after my departure fierce wolves will come in among you, not sparing the flock; and from among your own selves will arise men speaking twisted things, to draw away the disciples after them. Therefore be alert, remembering that for three years I did not cease night or day to admonish every one with tears. (Acts 20:29–31).
False teachers (wolves) will "come in" from the outside to harm the flock. People from the inside ("among your own selves") will also arise and teach "twisted things." And they will draw people into their destructive theology. So "be alert." Know the signs from holy Scripture. Be quick to listen, slow to speak, and cautious when you call someone a "false teacher". But nevertheless do so when necessary to protect others and for the sake of the truth.