fanatic

from The American Heritage® Dictionary of the English Language, 5th Edition.

• noun A person marked or motivated by an extreme, unreasoning enthusiasm, as for a cause.
• adjective Fanatical.

from The Century Dictionary.

• Same as fanatical.
• noun A person affected by zeal or enthusiasm, particularly on religious subjects; one given to wild and extravagant notions of religion.

from the GNU version of the Collaborative International Dictionary of English.

• noun A person affected by excessive enthusiasm, particularly on religious subjects; one who indulges wild and extravagant notions of religion.
• adjective Pertaining to, or indicating, fanaticism; extravagant in opinions; ultra; unreasonable; excessively enthusiastic, especially on religious subjects

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• adjective Fanatical.
• adjective obsolete Showing evidence of possession by a god or demon; frenzied, over-zealous.
• noun A person who is zealously enthusiastic for some cause, especially in religion.

from WordNet 3.0 Copyright 2006 by Princeton University. All rights reserved.

• adjective marked by excessive enthusiasm for and intense devotion to a cause or idea
• noun a person motivated by irrational enthusiasm (as for a cause)