In some form, it served as the national or state religion of a significant portion of the Iranian people for many centuries. The religion first dwindled when the Achaemenid Empire was invaded by Alexander the Great, after which it collapsed and disintegrated and it was further gradually marginalized by Islam from the 7th century onwards with the decline of the Sassanid Empire. The political power of the pre-Islamic Iranian dynasties lent Zoroastrianism immense prestige in ancient times, and some of its leading doctrines were adopted by other religious systems. It has no major theological divisions (the only significant schism is based on calendar differences), but it is not uniform. Modern-era influences have a significant impact on individual and local beliefs, practices, values and vocabulary, sometimes merging with tradition and in other cases displacing it.
The Oxford English Dictionary attests use of the term Zoroastrianism in 1874 in Archibald Sayce's Principles of Comparative Philology. The first surviving reference to Zoroaster in English scholarship is attributed to Thomas Browne (1605–1682), who briefly refers to the prophet in his 1643 Religio Medici. The Oxford English Dictionary records 1743 (Warburton, Pope's Essay) as the earliest reference to Zoroaster. However, his image is identified in Raphael's "School of Athens" by Giorgio Vasari in 1550, so knowledge of his philosophy had evidently percolated into the Italian Renaissance.
The term Mazdaism /ˈmæzdə.ɪzəm/ is a typical 19th century construct, taking Mazda- from the name Ahura Mazda and adding the suffix -ism to suggest a belief system. The March 2001 draft edition of the Oxford English Dictionary also records an alternate form, Mazdeism, perhaps derived from the French Mazdéisme, which first appeared in 1871. The Zoroastrian name of the religion is Mazdayasna, which combines Mazda- with the Avestan language word yasna, meaning "worship, devotion".
In English, an adherent of the faith commonly refers to himself or herself as a Zoroastrian or as a Zarathustrian. An older, but still widespread expression is Behdin, meaning "follower of Daena", for which "Good Religion" is one translation. In the Zoroastrian liturgy, the term Behdin is also used as a title for an individual who has been formally inducted into the religion in a Navjote ceremony.
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Zoroastrians believe that there is one universal, transcendent, supreme god, Ahura Mazda, or the 'Wise Lord'.(Ahura means 'Being' and Mazda means 'Mind' in Avestan language). Zoroaster keeps the two attributes separate as two different concepts in most of the Gathas and also consciously uses a masculine word for one concept and a feminine for the other, as if to distract from an anthropomorphization of his divinity. Some Zoroastrians claim Ahura Mazda as the uncreated Creator to whom all worship is ultimately directed, thereby formulating a panentheistic faith