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.