zealot

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

• noun One who is zealous, especially excessively so.
• noun A fanatically committed person.
• noun A member of a Jewish movement of the first century AD that fought against Roman rule in Palestine as incompatible with strict monotheism.

from The Century Dictionary.

• noun One who is zealous or full of zeal; one carried away by excess of zeal; an immoderate partizan: generally in a disparaging sense.
• noun [capitalized] One of a fanatical sect or party (the Zelotæ) among the Jews of Palestine under Roman dominion, who on account of their excesses in behalf of the Mosaic law were also called Sicarii or Assassins.

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

• noun One who is zealous; one who engages warmly in any cause, and pursues his object with earnestness and ardor; especially, one who is overzealous, or carried away by his zeal; one absorbed in devotion to anything; an enthusiast; a fanatical partisan.

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

• noun One who is zealous, one who is full of zeal for his own specific beliefs or objectives, usually in the negative sense of being too passionate; a fanatic
• noun historical A member of a radical, warlike, ardently patriotic group of Jews in Judea, particularly prominent in the first century, who advocated the violent overthrow of Roman rule and vigorously resisted the efforts of the Romans and their supporters to convert the Jews.
• noun historical A member of an anti-aristocratic political group in Thessalonica from 1342 until 1350.

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

• noun a fervent and even militant proponent of something
• noun a member of an ancient Jewish sect in Judea in the first century who fought to the death against the Romans and who killed or persecuted Jews who collaborated with the Romans