Zoroastrianism in Iran in the context of "List of countries by Zoroastrian population"

Zoroastrianism, also called Mazdayasna or Behdin, is an Iranian religion centred on the Avesta and the teachings of Zarathushtra Spitama, who is more commonly referred to by the Greek translation, Zoroaster (Greek: Ζωροάστρις Zōroastris). Among the world's oldest organized faiths, its adherents exalt an uncreated, benevolent, and all-wise deity known as Ahura Mazda (𐬀𐬵𐬎𐬭𐬀⸱𐬨𐬀𐬰𐬛𐬁), who is hailed as the supreme being of the universe. Opposed to Ahura Mazda is Angra Mainyu (𐬀𐬢𐬭𐬀⸱𐬨𐬀𐬌𐬥𐬌𐬌𐬎), who is personified as a destructive spirit and the adversary of all things that are good. As such, the Zoroastrian religion combines a dualistic cosmology of good and evil with an eschatological outlook predicting the ultimate triumph of Ahura Mazda over evil. Opinions vary among scholars as to whether Zoroastrianism is monotheistic, polytheistic, henotheistic, or a combination of all three. Zoroastrianism shaped Iranian culture and history, while scholars differ on whether it significantly influenced ancient Western philosophy and the Abrahamic religions, or gradually reconciled with other religions and traditions, such as Christianity and Islam.

Originating from Zoroaster's reforms of the ancient Iranian religion, Zoroastrianism began during the Avestan period (possibly as early as the 2nd millennium BCE), but was first recorded in the mid-6th century BCE. For the following millennium, it was the official religion of successive Iranian polities, beginning with the Achaemenid Empire, which formalized and institutionalized many of its tenets and rituals, and ending with the Sasanian Empire, which revitalized the faith and standardized its teachings. In the 7th century CE, the rise of Islam and the ensuing Muslim conquest of Iran marked the beginning of the decline of Zoroastrianism. The persecution of Zoroastrians by the early Muslims in the nascent Rashidun Caliphate prompted much of the community to migrate to the Indian subcontinent, where they were granted asylum and became the progenitors of today's Parsis. Once numbering in the millions, the world's total Zoroastrian population is estimated to comprise between 110,000 and 120,000 people, with most of them residing either in India (50,000–60,000), in Iran (15,000–25,000), or in North America (22,000). The religion is thought to be declining due to restrictions on conversion, strict endogamy, and low birth rates.

The Avesta is the text corpus of religious literature of Zoroastrianism. All its texts are composed in the Avestan language and written in the Avestan alphabet. Modern editions of the Avesta are based on the various manuscripts that have survived in India and Iran.

The individual texts of the Avesta were originally oral compositions. They were composed over a long period of several centuries during the Avestan period (possibly ranging from 15th century BCE – 4th century BCE). The written transmission began much later during the Sasanian era (224 to 651 CE), with the creation of the Avestan alphabet. The resulting texts were then compiled into the multi-volume edition of the Sasanian Avesta. This edition was lost sometime after the 10th century CE, and only a small portion of it has survived, scattered across a number of individual manuscript traditions. The oldest surviving fragment of such a manuscript dates to 1323 CE.