【Enthusiast】 ,  【fanatic】 ,  【zealot】 ,  【bigot】  denote a person who manifests excessive ardor, fervor, or devotion in his attachment to some cause, idea, party, or church.

【Enthusiast】 commonly denotes a person of keen and ardent interests and may carry either favorable or unfavorable connotations (as of mental or spiritual vitality or of a subordination of judgment to enthusiasm).

In earlier use and still in historical works the term applies particularly to a preacher, a member of a religious sect, or, sometimes, a poet who claims to be immediately inspired or who outwardly manifests signs (as rapture, madness, or intense emotionalism) associated with divine inspiration or possession by a god. In such context the term has been applied more or less contemptuously to a member of one of the strongly evangelical sects that arose in the seventeenth and eighteenth centuries.

【Fanatic】 , even more than  【enthusiast】 , carries a hint of madness or irrationality. In contrast to  【enthusiast】 , however, the term suggests extreme monomaniac devotion and a concentration of attention, sometimes on the end to be gained but, possibly more often, on the chosen means to one's end regardless of the real value of that end.  【Fanatic】 , therefore, in distinction from  【enthusiast】 , connotes determination, often silent determination, and an uncompromising temper.

【Zealot】 often implies fanaticism; it suggests ardent devotion, but it distinctively emphasizes vehement activity in the service of one's cause, party, or church. It may or may not connote blinding partisanship, but it usually suggests jealous vigilance in protecting one's beliefs or institutions.

【Bigot】  implies obstinate, often blind, devotion to one's own (especially religious) beliefs or opinions; as compared with  【fanatic】  and  【zealot】 , the term implies dogged intolerance and contempt for those who do not agree, rather than enthusiasm or zeal.