The preacher better be right...

Sin is mentioned a lot in the Bible, but perhaps the second most written about topic in the Bible is "false teachers."  Who is a false teacher?  Is he someone who just doesn't know, but is not trying to deceive?  He he some one getting a six figure pay check and he is telling the flock they need to put more in the offering?  Or, is he an agent for the devil, and appears to be an angel of light, being that deceptive?