What is a "False Teacher"?

In talking to people I realize there are varying opinions as to what a false teacher is. Is it the same as a false apostle or false doctrine? One thing that strikes me as odd is we almost always agree on these terms when it applies to worldly things. For instance, if Hitler gave his officers an order and another officer countered that with a different order, what would be the result? I remember when Saddam Husein took over Iraq. He walked into a government meeting and declared he was now THE leader. When a man stood up to oppose him, Saddam had him shot. No one else stood against him or even questioned his authority again. That is what "absolute" authority means, no one can say anything different than what you say.  It's as if these men were Kings.

    Mathew 28:18  "And Jesus came up and spoke to them, saying, “All authority  has been given to Me in heaven and on earth." 

  Jesus is a King, our Spiritual King, the King of Kings. When He tells us things it is not just for our listening pleasure, we are to listen and obey.  

    Jeremiah 14:14   "Then the Lord said to me, “The prophets are prophesying falsehood in My name. I have neither sent them nor commanded them nor spoken to them; they are prophesying to you a false vision, divination, futility and the deception of their own minds."

Paul says in 2 Corinthians 11:26    I have been on frequent journeys, in dangers from rivers, dangers from robbers, dangers from my countrymen, dangers from the Gentiles, dangers in the city, dangers in the wilderness, dangers on the sea, dangers among false brethren;

What about Galatians 2:4     But it was because of the false brethren secretly brought in, who had sneaked in to spy out our liberty which we have in Christ Jesus, in order to bring us into bondage.

Ok, one more, Galatians 1:8    But even if we, or an angel from heaven, should preach to you a gospel contrary to what we have preached to you, he is to be accursed!

You get the idea. There are a lot more verses telling us we are not to teach anything other than what the Apostles taught and they taught only what has been taught by Jesus and had been brought to their remembrance by the Holy Spirit. The narrow idea that a false prophet is only one who didn't believe in Jesus and the resurrection does not taking in account everything Jesus taught. The Word of God teaches a false teacher, prophet or even false doctrine comprises anyone and or anything that teaches one word different than what the Bible teaches. This is difficult to accept because it means there are a lot of people talking about God who are in fact false teachers.