As Zoroastrianism spread throughout the Persian Empire it took on some prominent modifications. The primary cause for this can be understood in the context of the religion itself and the religious disposition of the people that came in contact with it, both outside and within Iran. Zoroastrianism in its purest form rejected all gods but Ahura-Mazda - it was a strict monotheism. This, however, was not tolerated and accepted easily by the peoples so used to more corporeal deities closer to them - in the fields, the sky, in nature - and as such Zoroastrianism's strict monotheistic position was ameliorated: the ancient Aryan gods were raised to regain some former glory as mediators and angels of god, pagan rituals were re-introduced, and characteristics from gods of other lands were fused with the aboriginal Iranian deities such that the people were 'closer' to the divine presence.

This synthesis with polytheism and, indeed, corruption of monotheism was not unique to Zoroastrianism; it could be observed in other cases as well, such as the Catholic raising of saints unto near deitific proportions as veritable mediators between god and themselves, the constant vissicitudes that ancient Israel experienced whenever even a strict henotheism was launched, the disintigration of what some scholars suggest was a proto-monotheism in ancient Sumer, and the wide chasm between the Egyptian priesthood, that accepted a vague monotheism, and the flagrant polytheism of the Egyptian peasantry.

Of the many gods to regain status in Iranian religious life was Mithra. Eventhough, formally, within the newer sections of the Avesta, surmised to have been composed much later by the deviant Magi of later Zoroastrianism, he was classified as subordinate to Ahura-Mazda, so as to maintain the veneer of monotheism, however, was attributed abilities that indeed equalled him in power and importance to that of Ahura-Mazda. Mithra's ancient status is "lord of light" was doctored into "lord of the heavenly light;" the sun becomes his chariot and he with one thousand ears and ten thousand eyes watches the entirety of the world below him - he can not be fooled. In accordance with the ancient etymological origin of his name, he is again revived as god of oaths and contracts, and punishes those who betray their good word.

He is defender of truth - this, idea is neatly adjoined to the understanding of Ahura-Mazda as the 'lord of truth and good' - and Mithra is understood is Ahura's agent and mediator. Because of Mithra's association with the sun, the force of life, he himself is associated with the force of life - the god that brings forth light, heat, life, and nature. Indeed, it is said that eventhough Ahura created Mithra as he created all else, he nevertheless made him as great as himself. As the maintainer of light and the sun, Mithra is understood as the enemy of all that is dark. He is, as such, given the position of warrior against the darkness leading the forces of light against the forces of dark in the name of Ahura as is seen in the hymns of the Avesta.

As Zoroastrianism spread, it clearly came into contact with the Chaldaeans. The Chaldaeans had long been recognized for that astrology and magickal practices and it is evident that Zoroastrianism, particularly in the case of the divinity Mithra, borrowed heavily from their pagan mysteries. The Iranian divinity of springs and streams - Ardvisura Anahita, Anaitis - was fused with the model of the Babylonian Ishtar and Belit. Henceforth she was depicted as a beautiful and strong woman, with prominent breasts, a golden crown of stars and golden raiment and was worshipped as the goddess of generation and sexual life. Under Artaxerxes II in the first half of the 4th century B.C.E. statues of Anaitis were erected in Persepolis, Ecbatana, Bactria, Susa, Babylon, Damascus and Sardis, and the festival of Mithra became the chief festival of empire. This can be confirmed by enscriptions by Artaxerxes II and Artaxeerxes III where Anaitis and Mithra were invoked side by side with Ahura-Mazda. Heretofore, this Zoroastrian faith shall be refered to as Mazdaism as by the time of Persian occupation of the middle east so many of the original precepts of Zoroaster had been subverted.

Mazdaism became firmly rooted in the southern Caucasus and southern Anatolia, where Mithra gained an even more prominent role. Indeed, by the 4th and 3rd century B.C.E., Mithraism can be termed as an independent religious classification. The relation between it and Mazdaism remained strong; however, as Ahura-Mazda slowly faded into the background, this relation as well weakened. It was in the regions aforementioned, where Mazdaism became most firmly rooted in the west - that is, west of Iran - that the structure of Mithraic worship took on its final transformations following a further Hellenization of the region when Alexander took power.