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1.
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.
by Type36 October 08, 2007
 
2.
Refers to an attorney who is willing to give the benefit of the doubt to his clients, or to one who often advocates for justified causes despite long odds of success.
The True Believer does not negotiate. Her clients are innocent. Innocent people do not plead guilty. There will be no plea here. This case is going to trial!
by Advocate512 March 13, 2012