Enthusiasm is intense enjoyment, interest, or approval. The word was originally used to refer to a person possessed by a god, or someone who exhibited intense piety.
The word originates from the Greek ἐνθουσιασμός from ἐν and θεός and οὐσία, meaning "possessed by [a] god's essence", applied by the Greeks to manifestations of divine possession, by Apollo (as in the case of the Pythia), or by Dionysus (as in the case of the Bacchantes and Maenads), the term enthusiasm was also used in a transferred or figurative sense. Socrates taught that the inspiration of poets is a form of enthusiasm. The term was confined to a belief in religious inspiration, or to intense religious fervour or emotion.
From this, a Syrian sect of the 4th century was known as the Enthusiasts. They believed that "by perpetual prayer, ascetic practices and contemplation, man could become inspired by the Holy Spirit, in spite of the ruling evil spirit, which the fall had given to him". From their belief in the efficacy of prayer, they were also known as Euchites.
Several Protestant sects of the 16th and 17th centuries were called enthusiastic. During the years that immediately followed the Glorious Revolution, "enthusiasm" was a British pejorative term for advocacy of any political or religious cause in public, i.e. fanaticism. Such "enthusiasm" was seen in the time around 1700 as the cause of the previous century's English Civil War and its attendant atrocities, and thus it was an absolute social sin to remind others of the war by engaging in enthusiasm. The Royal Society bylaws stipulated that any person discussing religion or politics at a Society meeting was to be summarily ejected for being an "enthusiast." During the 18th century, popular Methodists such as John Wesley or George Whitefield were accused of blind enthusiasm, a charge against which they defended themselves by distinguishing fanaticism from "religion of the heart."