Truth

I'm writing a story currently and it's brought up an interesting question. What do we really know?
It's about (without saying too much) a girl who was raised to believe that she has a demon inside of her vying for control. She was taught that every time she made a mistake or sinned, it gave the demon a greater control over her.
Not a hugely fascinating or deep idea, but while going through some characterizations with her I realized the dilemma. We only know what we are taught.
The Laminates in ancient America were taught by the "wicked traditions of their fathers" but that is what they knew to be correct. How could they be blamed?
What if we don't know what we think we know?
I've had a few different discussions on the subject of "truth." Can you have a knowledge of something that is not true? One of the men I was arguing with said you cannot, but I think that you can. Hundreds of years ago people knew that the earth was flat. They knew it. It was the knowledge of the time. There's nothing wrong with that, their idea was just not correct.
Science is a constant cycle of that same idea. We have our theories and laws until we learn more and gain a deeper understanding that proves our old ideas to be false. So we revise the textbooks and move on.
This causes me to wonder... what do we know now that isn't true? I'm sure there are the theories of the universe, how it was created and things of that nature that we just don't have any clue about. But what about our every day things? We have our basic ideas of what is right and what is wrong, but so did the Laminates. Now, I have my religion, obviously, and I believe in the things that I choose to believe. But assuming that one religion is more correct than another is to say that what other people know to be "truth" is incorrect.
It's sort of a scary thought, isn't it?
What do we know? What is truth? Maybe the earth really is flat. Who knows?