What do you call a person who believes something that is not true?

You just call it a real person. Everyone believes in something that they think is true even if other people don't. Or you could just call it a "____-believer." I don't know. This question has a lot of answers,but I guess that my first answer was right("you just call it a real person" one).

Ignorant, gullible, brainwashed, naive, pig-headed, hard-headed, Oblivious, mindless, dense