Enthusiast

En*thu"si*ast (?), n. [Gr. ?: cf. F. enthousiaste.] One moved or actuated by enthusiasm; as: (a) One who imagines himself divinely inspired, or possessed of some special revelation; a religious madman; a fanatic. (b) One whose mind is wholly possessed and heated by what engages it; one who is influenced by a peculiar; fervor of mind; an ardent and imaginative person.

Enthusiasts soon understand each other.
W. Irving.

Syn. -- Visionary; fanatic; devotee; zealot.