But there were false prophets also among the people, even as there shall be false teachers among you, who privily shall bring in damnable heresies, even denying the Lord that bought them, and bring upon themselves swift destruction. And many shall follow their pernicious ways; by reason of whom the way of truth shall be evil spoken of. And through covetousness shall they with feigned words make merchandise of you: whose judgment now of a long time lingereth not, and their damnation slumbereth not.
- 2 Peter 2:1-3 (KJV)
Most people think this only happened 2,000 years ago, but no, it's happening today.
False teachers are wolves.
Beware of false prophets, which come to you in sheep's clothing, but inwardly they are ravening wolves. Ye shall know them by their fruits. Do men gather grapes of thorns, or figs of thistles? Even so every good tree bringeth forth good fruit; but a corrupt tree bringeth forth evil fruit. A good tree cannot bring forth evil fruit, neither can a corrupt tree bring forth good fruit. Every tree that bringeth not forth good fruit is hewn down, and cast into the fire. Wherefore by their fruits ye shall know them.
- Matthew 7:15-20 (KJV)
False teachers can be hard to distinguish.
And no marvel; for Satan himself is transformed into an angel of light. Therefore it is no great thing if his ministers also be transformed as the ministers of righteousness; whose end shall be according to their works.
- 2 Corinthians 11:14-15 (KJV)
Most people don't fully understand exactly how dangerous false teachers are.
The Heretic is the most prominent and perhaps the most dangerous of the false teachers. Peter warned against him in his second letter. "But false prophets also arose among the people, just as there will be false teachers among you, who will secretly bring in destructive heresies, even denying the Master who bought them, bringing upon themselves swift destruction" (2 Peter 2:1). The Heretic is the person who teaches what blatantly contradicts an essential teaching of the Christian faith. He is a gregarious figure, a natural leader teaching just enough truth to mask his deadly error. Yet in denying the faith and celebrating what is false, he leads his followers from the safety of orthodoxy to the peril of heresy.
- Tim Challies
The Bible is quite clear what to do with heretics.
A man that is an heretick after the first and second admonition reject; Knowing that he that is such is subverted, and sinneth, being condemned of himself.
- Titus 3:10-11 (KJV)
In other words, warn them once, twice, and then have nothing further to do with them (avoid them where possible).
Now I beseech you, brethren, mark them which cause divisions and offences contrary to the doctrine which ye have learned; and avoid them.
- Romans 16:17 (KJV)
The Bible specifically warns about the doctrines of demons.
Now the Spirit speaketh expressly, that in the latter times some shall depart from the faith, giving heed to seducing spirits, and doctrines of devils; Speaking lies in hypocrisy; having their conscience seared with a hot iron; Forbidding to marry, and commanding to abstain from meats, which God hath created to be received with thanksgiving of them which believe and know the truth. For every creature of God is good, and nothing to be refused, if it be received with thanksgiving: For it is sanctified by the word of God and prayer.
- 1 Timothy 4:1-5 (KJV)
You need to know what the Bible teaches to understand who is a false teacher, etc.
False teachers are very dangerous, and can easily lead you in the wrong direction.
Test the spirits, and simply ask people who Jesus is to them.
Do not underestimate them.