In my association with fundamentalist churches I constantly heard about false teachers, religious instructors who did not teach the truth. Generally it was reiterated that it didn't matter whether the teachers were sincere in their false beliefs or not; the result of heeding them was the same, eternal damnation.
In retrospect, maybe I should have been more interested in what caused this prevalence of false teaching. Some probably believed that false doctrine was so widespread because Satan was actively spreading misunderstanding, using individuals who were willful perpetrators of religious error. Others may have assumed that sincere people often simply could not understand the truth correctly because they did not study the scriptures enough or accepted traditional religious instruction, even if non-biblical.
If the scourge of false teaching was the result of the work of Satan, inspiring evil men to sell religious lies to the gullible, that made the true instructors sound particularly brave and noble, surrounded as they were by a vast crowd of devil worshippers. However, if the myriad differences in teaching among the religious elements was simply a matter of sincerely held differences of opinion and not evidence of some sinister supernatural forces, then those who claimed to hold and teach the truth had a much greater challenge. They needed to explain why their truth was not more self evident, more persuasive, more powerfully demonstrable. An ineffective divine truth, one easily misunderstood and/or rejected hardly seems God ordained. The working of the Creator should be completely reliable and effective in their divine purpose.
One easily comes to suspect that all the hoopla about false teachers in the jargon of fundamentalism is largely a smokescreen, an attempt to explain away the inexplicable. When most people don't agree with them, and they think everyone must, then the evident facts fly in the face of their doctrine unless God can be diminished in motive, power, or will, thereby creating an excuse. Any who don't accept such a diminished god will of necessity be labeled false teachers.