Zoroastrianism combines elements of monotheism and dualism. Some modern scholars believe that Zoroastrianism had a large influence on Judaism and Manichaeism, and thus indirectly influenced Christianity and Islam.
The holy book of Zoroastrianism is the Avesta. Of the Avesta only the Gathas (the hymns) are attributed to Zoroaster.
Ahura Mazda (literally: "the Wise Lord" like the Sanskrit "Asura Medha"; later transcription: Ohrmazd, Ormazd or Ormus) is revered and worshipped by Zoroastrians as the good God. Opposed to Ahura Mazda stands Ahriman (Angra Mainyu), who in some traditions is Ohrmazd's twin brother, in others the twin of Spenta Mainyu; modern Zoroastrianism considers itself monotheistic and looks upon Spenta Mainyu and Angra Mainyu as allegorical personifications. According to Zoroastrianism, the earth was created by Ormazd as a battlefield to fight Ahriman (where Ohrmazd is destined to win approximately 3000 years after Zoroaster, that is, circa AD 2400). Human beings have free will to choose between Ohrmazd and Ahriman, however once this choice is made it is impossible or nearly impossible to change. Those who align with Ohrmazd are believed to go directly to Heaven after death or resurrection (depending on the tradition), whereas those who align with Ahriman go to Hell for a period of time before then going on to Heaven. Unlike Manichaeism, Zoroastrianism does not associate matter with evil. On the contrary, material pursuits such as raising a family and creating wealth are considered to aid Ohrmazd. "Good thoughts, good words, good deeds" is a common slogan.
However, Zoroastrianism is not simply the purely ethical religion it may at first seem. Purification rituals are important, and Zoroastrians practise sacrifice as well as confession. Indeed a religious Zoroastrian must constantly be involved in a meticulous struggle against the contamination of death (which is associated with Ahriman) and of the many other causes of defilement, and against the threat - even in sleep - of demons. Fire is an important religious symbol, and once started a ritual fire must be kept continually burning. The dead are not buried or cremated, but left for vultures to eat in special temples for that purpose.
Small Zoroastrian communities survive in Iran and in India, totalling 140,000 followers. Iranian Zoroastrians are called Gabars (a name deriving from the Arabic word kaffir meaning infidel), but this is a pejorative term. Some Zoroastrians in Yazd and Kerman still speak an Iranian language distinct from Farsi. They call their language Dari (not to be confused with the Dari of Afghanistan). Their language is also called Gabri or Behdinan. Sometimes their language is named for the cities in which they are spoken, Yazdi or Kermani. Other small Zoroastrian communities exist in large cities in the United States and Canada.
One of the most famous Zoroastrians is the late Freddie Mercury, the frontman of the group Queen. He was given a traditional Zoroastrian funeral after he died of AIDS on the 24th of November, 1991. Symphonic conductor Zubin Mehta is also a Zoroastrian, as well as the Tata industrial family in India.
In the 7th century, the Zoroastrian Sassanid dynasty was conquered by Muslim Arabs, and Zoroastrians were awarded the status of People of the Book by the Caliph Omar, although some practices contrary to Islam were prohibited, such as sibling marriages.
In the 8th century, Zoroastrians fled to India in large numbers, where they were given refuge by Jadi Rana, king of Sanjan (the modern-day province of Gujarat) on condition that they abstain from missionizing local Hindus and marry only in their community. Although these strictures are centuries old, Parsis of the 21st century still do not accept converts and are endogamous. The Parsis of India speak a Gujarati dialect.
The earliest English references to Zoroaster and the Zoroastrian religion occur in the writings of the encyclopaedist Sir Thomas Browne.