noun

a person filled with excessive and single-minded zeal, especially for an extreme religious or political cause.

religious fanatics

zealot,extremist,militant,dogmatist,devotee,sectarian,bigot,chauvinist,partisan,radical,diehard,ultra,activist,apologist,adherent,visionary,maniac,crank,freak,wackadoo,wackadoodle

adjective

filled with or expressing excessive zeal.

his eyes had a fanatic iciness