A person motivated by irrational enthusiasm (as for a cause):
— A fanatic is one who can't change his mind and won't change the subject.

synonym: fiend.

Roget 988: impiety; sin etc. 945; irreverence; profaneness etc. adj.; profanity, profanation; blasphemy, desecration, sacrilege; scoffing etc. v.. [feigned piety] hypocrisy etc. (falsehood) 544; pietism, cant, pious fraud; lip devotion, lip service, lip reverence; misdevotion^†, formalism, austerity; sanctimony, sanctimoniousness etc. adj.; pharisaism, precisianism^†; sabbatism^†, sabbatarianism^†; odium theologicum [Lat.], sacerdotalism^†; bigotry etc. (obstinacy) 606, (prejudice) 481; blue laws.    hardening, backsliding, declension, perversion, reprobation.    sinner etc. 949; scoffer, blasphemer; sacrilegist^†; sabbath breaker; worldling; hypocrite etc. (dissembler) 548; Tartufe^†, Mawworm^†.    bigot; saint [Iron.]; Pharisee; sabbatarian^†, formalist, methodist, puritan, pietist^†, precisian^†, religionist, devotee; ranter, fanatic, juramentado^†.    the wicked, the evil, the unjust, the reprobate; sons of men, sons of Belial, the wicked one; children of darkness.    ... show more

Roget 984: heterodoxy; error etc. 495; false doctrine, heresy, schism; schismaticism^†, schismaticalness; recusancy, backsliding, apostasy; atheism etc. (irreligion) 989^†.    bigotry etc. (obstinacy) 606; fanaticism, iconoclasm; hyperorthodoxy^†, precisianism^†, bibliolatry^†, sabbatarianism^†, puritanism; anthropomorphism; idolatry etc. 991; superstition etc. (credulity) 486; dissent etc. 489.    sectarism^†, sectarianism; noncomformity^†; secularism; syncretism^†.    [religious sects.] protestantism, Arianism^†, Adventism, Jansenism, Stundism^†, Erastianism^†, Calvinism, quakerism^†, methodism, anabaptism^†, Puseyism, tractarianism^†, ritualism, Origenism, Sabellianism, Socinianism^†, Deism, Theism, materialism, positivism, latitudinarianism etc. High Church, Low Church, Broad Church, Free Church; ultramontanism^†; papism, papistry; monkery^†; papacy; Anglicanism, Catholicism, Romanism; popery, Scarlet Lady, Church of Rome, Greek Church.    paganism, heathenism, ethicism^†; mythology; polytheism, ditheism^†, tritheism^†; dualism; heathendom^†.    Judaism, Gentilism^†, Islamism, Islam, Mohammedanism, Babism^†, Sufiism, Neoplatonism, Turcism^†, Brahminism, Hinduism, Buddhism, Sabianism, Gnosticism, Hylotheism^†, Mormonism; Christian Science.    heretic, apostate, antichrist^†; pagan, heathen; painim^†, paynim^†; giaour^†; gentile; pantheist, polytheist; idolator.    schismatic; sectary, sectarian, sectarist^†; seceder, separatist, recusant, dissenter; nonconformist, nonjuror^†.    bigot etc. (obstinacy) 606; fanatic, abdal^†, iconoclast.    latitudinarian, Deist, Theist, Unitarian; positivist, materialist; Homoiousian^†, Homoousian^†, limitarian^†, theosophist, ubiquitarian^†; skeptic etc. 989.    Protestant; Huguenot; orthodox dissenter, Congregationalist, Independent; Episcopalian, Presbyterian; Lutheran, Calvinist, Methodist, Wesleyan; Ana^†, Baptist; Mormon, Latter-day Saint^†, Irvingite, Sandemanian, Glassite, Erastian; Sublapsarian, Supralapsarian^†; Gentoo, Antinomian^†, Swedenborgian^†; Adventist^†, Bible Christian, Bryanite, Brownian, Christian Scientist, Dunker, Ebionite, Eusebian; Faith Curer^†, Curist^†; Familist^†, Jovinianist, Libadist^†, Quaker, Restitutionist^†, Shaker, Stundist, Tunker &c.; ultramontane; Anglican^†, Oxford School; tractarian^†, Puseyite, ritualist; Puritan.    Catholic, Roman, Catholic, Romanist, papist.    Jew, Hebrew, Rabbinist, Rabbist^†, Sadducee; Babist^†, Motazilite; Mohammedan, Mussulman, Moslem, Shiah, Sunni, Wahabi, Osmanli.    Brahmin^†, Brahman^†; Parsee, Sufi, Buddhist; Magi, Gymnosophist^†, fire worshiper, Sabian, Gnostic, Rosicrucian etc. ... show more

Roget 504: madman, lunatic, maniac, bedlamite^†, candidate for Bedlam, raver^†, madcap, crazy; energumen^†; automaniac^†, monomaniac, dipsomaniac, kleptomaniac; hypochondriac etc. (low spirits);; crank, Tom o'Bedlam.    dreamer etc. 515; rhapsodist, seer, highflier^†, enthusiast, fanatic, fanatico [Sp.]; exalte [Fr.]; knight errant, Don Quixote.    idiot etc. 501.    ... show more

Roget 606: obstinateness etc. adj.; obstinacy, tenacity; cussedness [U.S.]; perseverance etc. 604.1; immovability; old school; inflexibility etc. (hardness) 323; obduracy, obduration^†; dogged resolution; resolution etc. 604; ruling passion; blind side.    self-will, contumacy, perversity; pervicacy^†, pervicacity^†; indocility^†.    bigotry, intolerance, dogmatism; opiniatry^†, opiniativeness; fixed idea etc. (prejudgment) 481; fanaticism, zealotry, infatuation, monomania; opinionatedness opinionativeness^†.    mule; opinionist^†, opinionatist^†, opiniator^†, opinator^†; stickler, dogmatist; bigot; zealot, enthusiast, fanatic.    ... show more

Dutch: dweper, fanaat, fanaticus, fanatiekeling, freak, gedrevene
Polish: fanatyk, maniak

Marked by excessive enthusiasm for and intense devotion to a cause or idea.

synonyms: fanatical, overzealous, rabid.

Roget 515: imagined etc. v.; ben trovato [It]; air drawn, airbuilt^†.    imagining etc. v., imaginative; original, inventive, creative, fertile.    romantic, high flown, flighty, extravagant, fanatic, enthusiastic, unrealistic, Utopian, Quixotic.    ideal, unreal; in the clouds, in nubibus [Lat.]; unsubsantial^† etc. 4; illusory etc. (fallacious) 495.    fabulous, legendary; mythical, mythic, mythological; chimerical; imaginary, visionary; notional; fancy, fanciful, fantastic, fantastical^†; whimsical; fairy, fairy-like; gestic^†.    ... show more

Polish: fanatyczny, nawiedzony