Can false teachers come into a local church without the average church member knowing it?

Les teaches that false teachers can come into a local church, and they are so deceptive that the average church member just doesn't know what they're doing. The text describes them as having crept in unawares, men ordained of old to this condemnation, ungodly men who turn the grace of God into lasciviousness and deny the only Lord God, our Lord Jesus Christ. Les notes there are a lot of them doing this, and then the text explains how these false teachers operate: they speak evil of things they know not. Their way is the way of Cain—no faith—and they run greedily after the error of Balaam for reward. Balaam thought that if he could get Israel to lose her separated position with Jehovah and sin openly, God would curse them; Les says he succeeded, and that was the error.