'''Zoroastrianism''' (Avestan '''Daēnā Vañuhi''' ''the good religion'')<ref>Jafari, A., ''The Good Religion and Zoroastrianism'', ([http://www.zoroastrian.org/articles/The_Good_Religion_and_Zoroastrianism.htm ''LINK'']); accessed March 05, 2007.</ref><ref>CAIS, ''An Introduction to Daenâ Vanuhi (The Good Religion) of Asho Zarathushtra'', ([http://www.cais-soas.com/CAIS/Religions/iranian/Zarathushtrian/introduction.htm ''LINK'']); accessed March 05, 2007.</ref> is the religion and philosophy based on the teachings ascribed to the prophet Zoroaster (Zarathustra, Zartosht). '''Mazdaism''' is the religion that acknowledges the divine authority of [[Ahura ]] Mazda, proclaimed by Zoroaster to be the one uncreated Creator of all (God).

Perhaps more importantly, ''The Histories'' is a primary source of information on the early period of the Achaemenid Empire (648 BCE–330 BCE), in particular with respect to the role of the Magi. According to Herodotus i.101, the "Magi" were the sixth tribe of the Medians (until the unification of the Persian empire under Cyrus the Great, all Iranians were referred to as Mede or Mada by the peoples of the Ancient World), who appear to have been the priestly caste of the Mesopotamian-influenced branch of Zoroastrianism today known as "[[Zurvanism]]", and who wielded considerable influence at the courts of the Medes emperors.

Whether [[Cyrus the Great|Cyrus II ]] was a Zoroastrian is subject to debate. It did however influence him to the extent that it became the non-imposing religion of his empire, and its beliefs would later allow Cyrus to free the Jews from captivity (and allow them to return to Judea) when the emperor took Babylon in 539 BCE. Whether Darius I, though certainly a devotee of Ahura Mazda (as attested to several times in the Behistun inscription), was a follower of Zoroaster has not been conclusively established, since a devotion to Ahura Mazda was (at the time) not necessarily an indication of an adherence to Zoroaster's teaching.

When the Sassanid dynasty came into power in 228, they aggressively promoted the [[Zurvanite ]] form of Zoroastrianism and in some cases persecuted [[Christianity|Christians]] and Manichaeism. When the Sassanids captured territory, they often built fire temples there to promote their religion. The Sassanids were suspicious of Christians not least because of their perceived ties to the Christian Roman Empire. Thus, those Christians loyal to the List of Patriarchs of Babylon — which had broken with Roman Christianity when the latter condemned Nestorianism — were tolerated and even sometimes favored by the Sassanids. Nestorians lived in large numbers in Mesopotamia and Khuzestan during this period.

In the centuries following the fall of the Sassanid Empire, Zoroastrianism began to gradually return to the form it had had under the Achaemenids, and no evidence of what is today called the "[[Zurvan ]] Heresy" exists beyond the 10th century CE. (Boyce, 2002) Ironically, it was [[Zurvanism ]] and [[Zurvan]]-influenced texts that first reached the west, leading to the supposition that Zoroastrianism was a religion with two deities: [[Zurvan ]] and Ahura Mazda (the latter being opposed by Angra Mainyu).

Through accumulation, several other beliefs were introduced to the religion, that in some instances supersede those expressed in the Gathas. In the late 19th century, the moral and immoral forces came to be represented by ''Spenta Mainyu'' and its Satanic antithesis ''Angra Mainyu'', the 'good spirit' and 'evil spirit' emanations of Ahura Mazda respectively. Although the names are old, this opposition is a modern western-influenced development popularized by Martin Haug in the 1880s, and was in effect a realignment of the precepts of [[Zurvanism ]] ([[Zurvanite ]] Zoroastrianism), which had invented a ''third'' deity, ''[[Zurvan]]'', in order to explain a mention of twinship (''Yasna'' 30.3) between the moral and immoral. Although [[Zurvanism ]] had died out by the 10th century, the critical question of the "twin brothers" mentioned in ''Yasna'' 30.3 remained, and Haug's explanation provided a convenient defence against Christian missionaries who disparaged the Parsis (Indian Zoroastrians) for their 'dualism'. Haug's concept was subsequently disseminated as a Parsi interpretation, thus corroborating Haug's theory and the idea became so popular that it is now almost universally accepted as doctrine.

In addition, and strongly influenced by Babylonian and Akkadian practices, the Achaemenids popularized shrines and temples, hitherto alien forms of worship. In the wake of Achaemenid expansion, shrines were constructed throughout the empire and particularly influenced the role of [[Mithra]], Aredvi Sura Anahita, Vahram and Tishtrya, all of which, in addition to their original (proto-)Indo-Iranian functions, now also received Perso-Babylonian functions.

Some Zoroastrians believe in the future coming of a [[Messiah]]-like figure known as the [[Peshotan]]. This too is a modern syncretic development, and is frowned upon by more conservative Zoroastrians.

Few (if any) adherents remain in the Central Asian regions that were once considered the traditional stronghold of Zoroastrianism, i.e. Bactria (see also Balkh) which is in Northern Afghanistan, Sogdiana, Margiana, and other areas closest to Zoroaster's homeland.<!-- homeland/area definition see Boyce, 1975:chap 1. For "closest to Zoroaster's homeland", see Boyce's theory why Mazdaism survived but [[Zurvanism ]] did not 1957:308-309 -->