Cosmology
Zarathustra proclaimed that Ahura Mazda was the supreme creator and sustaining force of the universe, working in gētīg (the visible material realm) and mēnōg (the invisible spiritual and mental realm) through the Amesha Spenta, a class of seven divine entities that represent various aspects of the universe and the highest moral good. Emanating from Ahura Mazda is Spenta Mainyu (the Holy or Bountiful Spirit), the source of life and goodness, which is opposed by Angra Mainyu (the Destructive or Opposing Spirit), who is born from Aka Manah (evil thought). Angra Mainyu was further developed by Middle Persian literature into Ahriman (𐭠𐭧𐭫𐭬𐭭𐭩), Ahura Mazda's direct adversary.
Zoroastrian doctrine holds that, within this cosmic dichotomy, human beings have the choice between Asha (truth, cosmic order), the principle of righteousness or "rightness" that is promoted and embodied by Ahura Mazda, and Druj (falsehood, deceit), the essential nature of Angra Mainyu that expresses itself as greed, wrath, and envy. Thus, the central moral precepts of the religion are good thoughts (hwnata), good words (hakhta), and good deeds (hvarshta), which are recited in many prayers and ceremonies. Many of the practices and beliefs of ancient Iranian religion can still be seen in Zoroastrianism, such as reverence for nature and its elements, such as water (aban). Fire (atar) is held by Zoroastrians to be particularly sacred as a symbol of Ahura Mazda himself, serving as a focal point of many ceremonies and rituals, and serving as the basis for Zoroastrian places of worship, which are known as fire temples.
Zoroastrians treat Ahura Mazda as the supreme god, but believe in lesser divinities known as Yazata. These yazata ("good agents") include Anahita, Sraosha, Mithra, Rashnu, and Tishtrya. Yazata is the Avestan word for a Zoroastrian concept with a wide range of meanings but generally signifying (or used as an epithet of) a divinity. The term literally means "worthy of worship or veneration".
The religion of Iran is closely akin to that of northern India, and, hence, the people of the two lands probably had common ancestors who spoke a common Indo-Aryan language. The religion of those peoples has been reconstructed by means of common elements contained in the sacred books of Iran and India, mainly the Avesta and the Veda.
Zarathustra's religious philosophy divided the early Iranian gods of Proto-Indo-Iranian paganism into emanations of the natural world—the ahura and the daeva; the former class consisting of divinities to be revered and the latter class consisting of divinities to be rejected and condemned.
Both the Avesta and the Veda exhibit the same kind of polytheism with many of the same gods, notably the Indian Mitra (the Iranian Mithra), the cult of fire, sacrifice by means of a sacred liquor (Soma in India, Haoma in Iran), and other parallels. There is, moreover, a list of Indo-Iranian gods in a treaty concluded about 1380 bce between the Hittite emperor and the king of Mitanni. The list includes Mitra and Varuna, Indra, and the two Nāsatyas. All of these gods also are found in the Vedas but only the first one in the Avesta, except that Indra and Nāñhaithya appear in the Avesta as demons; Varuna may have survived under another name.
The Indo-Iranians appear to have distinguished from among their gods the Daiva (Indo-Iranian and Old Persian equivalent of Avestan Daeva and Sanskrit Deva, related to the Latin deus), meaning “heavenly,” and the Asura, a special class with occult powers. This situation was reflected in Vedic India; later on, Asura came to signify, in Sanskrit, a kind of demon, because of the baleful aspect of the Asura’s invisible power. In Iran the evolution must have been different: the Ahura were extolled to the exclusion of the Daeva, who were reduced to the rank of demons.
It is in the framework of the religion of the Ahura, hostile to the cult of the Daeva, that Zarathushtra’s message should be understood. He emphasized the central importance of his god, the wise Ahura (Ahura Mazda), by portraying him with an escort of entities, the powers of all the other gods, in an array against the forces of evil.
The moral dualism expressed in the opposition Asha–Druj (truth–falsehood) goes back at least to Indo-Iranian times, for the Veda knows it too, as rita-druh, though the contrast is not as sharply defined as in the Avesta. Between these two principles, the Twin Spirits made an ominous choice, the Bounteous One becoming in thoughts, words, and deeds a partisan of Asha, Ashavan, while the other became Dregvant, partisan of the Druj. After them it was the Daeva’s turn, and they all chose wrongly. Ever since, the Daeva have tried to corrupt each human being’s choice also.
Saoshyant is an Avestan-language term that literally means "one who brings benefit", and which is used in several different ways in Zoroastrian scripture and tradition. In particular, the expression is the proper name of the Saoshyant, an eschatological saviour figure who brings about Frashokereti, the final renovation of the world in which evil is finally destroyed.