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.
Billy Meier has his own little UFO cult of True Believers
True Believers can range in their beliefs from the spiritual (Tarot, Astrology, Chi, etc.) to political (Holocaust Denial, Jews did WTC, JFK was killed by the government, etc.) to medical (Homeopathy, Vaccine causes Autism, etc.) and on to almost any other system.