|“||A person who earnestly believes in something, despite overwhelming scientific evidence to the contrary. Usually used as a derogatory term to imply the lack of critical thinking and/or intelligence on part of the true believer.||”|
|“||For many people, the will to believe at times overrides the ability to think critically about the evidence for and against a belief. (...) One possible explanation for true-believer syndrome is that the belief satisfies an emotional need that is stronger than any other emotional need. Why some people have such a strong emotional need to believe in something that rational people recognize as false is perhaps unanswerable, but it is clear that the kind of beliefs we are discussing here are based on emotions and feelings, not reason and evidence.||”|
True Believers can believe anything that's weird and unreasonable, Astrology, Christian Fundamentalism, also a range of medical and political woo can appeal to a type of true believer. True Believers are fanatical and passionate about what they believe. 
Mainstream Christians aren't called True Believers though mainstream Christianity is also weird. It looks like to be labelled a True Believers a person has to believe something irrational that the majority don't believe.