Persians weren't monotheistic in Cyrus' time. People mistakenly project later Zoroastrian beliefs from the 6th century AD back onto earlier Iranians. Zoroastrianism didn't become a state religion of Persians till the Sassanian empire, most than 750 years after Cyrus. This was in the early Christian era, so Zoroastrian beliefs reflecting a belief in a supreme God, Ahura Mazda, could have actually been copied from Judaism or Christianity.
Re the Avesta:
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The text that is now extant represents only a fragment of what remained in the 9th century of the late Sasanian Avesta compiled under the direction of Khosrow I (531–579 ce). Summaries of the contents of the Sasanian Avesta show that it was an enormous collection containing texts in Avestan as well as in—and predominantly so— Pahlavi, the language of Sasanian Zoroastrianism. In spite of the relatively recent date of the existing Avesta, it contains matter of great antiquity, of which the Gathas (“Songs”) of the Prophet Zoroaster (Zarathustra; flourished before the 6th century bce) and much of the Yashts are among the oldest. The Gathas contain expressions of Zoroaster’s religious vision which, in many ways, is a complicated reinterpretation of inherited Iranian religious ideas. The Yashts are collections of verses dedicated to the various deities. Most of the Yashts, though touched up with Zoroastrian terminology and ideas, have little to do with anything specifically Zoroastrian. The gods invoked are essentially the gods of pre-Zoroastrian Iran.
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The earliest religious texts of the closely related Indo-Aryan speakers (principally the Rigveda) are indispensable for making historical reconstructions of the development of Iranian religion. The Rigveda, a collection of more than 1,000 hymns to various deities, can be dated to a period from approximately 1300 to 900 bce. Apart from the Achaemenian inscriptions, there is no secure evidence that religious compositions were reduced to writing until the late Arsacid or early Sasanian periods. Thus, unlike the other religions of the Middle East, the Iranian religions had no written texts in the ancient period. All religious “literature” was oral, in both composition and transmission.
britannica.com
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The student of Zoroastrianism is confronted by several problems concerning the religion’s founder. One question is what part of Zoroastrianism derives from Zoroaster’s tribal religion and what part was new as a result of his visions and creative religious genius. Another question is the extent to which the later Zoroastrian religion (Mazdaism) of the Sasanian period (ad 224–651) genuinely reflected the teachings of Zoroaster. A third question is the extent to which the sources—the Avesta (the Zoroastrian scriptures) with the Gathas (older hymns), the Middle Persian Pahlavi Books, and reports of various Greek authors—offer an authentic guide to Zoroaster’s ideas.A biographical account of Zoroaster is tenuous at best or speculative at the other extreme. The date of Zoroaster’s life cannot be ascertained with any degree of certainty. According to Zoroastrian tradition, he flourished “258 years before Alexander.” Alexander the Great conquered Persepolis, the capital of the Achaemenids, a dynasty that ruled Persia from 559 to 330 bc, in 330 bc. Following this dating, Zoroaster converted Vishtaspa, most likely a king of Chorasmia (an area south of the Aral Sea in Central Asia), in 588 bc. According to tradition, he was 40 years old when this event occurred, thus indicating that his birthdate was 628 bc.
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According to the sources, Zoroaster probably was a priest. Having received a vision from Ahura Mazda, the Wise Lord, who appointed him to preach the truth, Zoroaster apparently was opposed in his teachings by the civil and religious authorities in the area in which he preached. It is not clear whether these authorities were from his native region or from Chorasmia prior to the conversion of Vishtaspa. Confident in the truth revealed to him by Ahura Mazda, Zoroaster apparently did not try to overthrow belief in the older Iranian religion, which was polytheistic; he did, however, place Ahura Mazda at the centre of a kingdom of justice that promised immortality and bliss. |
