Baga vazraka Auramazdâ

The Achaemenid dynasty existed during a dynamic period in Iranian religion.  Prehistoric Iranians were polytheists like their proto-Indo-European predecessors.  They revered ancient gods such as Mithra, Anahita and Verethragna, who have counterparts in Hinduism.

Some time prior to the Achaemenid period, a priest named Zoroaster (Zarathustra) in eastern Iran or Afghanistan began to preach that there was only one true uncreated God, whom he called Ahura Mazda, the "Wise Spirit," and denounced many of the daevas (divinities) as malevolent beings unworthy of worship (roughly like demons in Christianity), such that this is what daeva came to mean.  He described his beliefs in a series of hymns and teachings which form the core of the Zoroastrian scriptures, the Avesta.

Today, Zoroastrianism is definitely monotheistic, though acknowledging lesser spirits or angels worthy of devotion (yazatas, from the root yaz-, "to worship/venerate"), among them many proto-Iranian former gods.  In the absence of such subtle distinctions in Achaemenid-period primary sources, it's not easy to judge whether Persians at that time were monotheists or polytheists.

Cyrus claimed in the Cyrus Cylinder to have been made king by Marduk, the patron god of Babylon.  But this is a political document addressing the adherents of Marduk, and can't necessarily be taken as expressing Cyrus' personal beliefs.

Baga vazraka Auramazdâ hya avam asmânam adâ hya imâm bum im adâ hya martiyam adâ hya šiyâtim adâ martiyahyâ hya Dârayavaum XŠyam...
"A great god is Ahuramazda, who created this Earth, who created yonder sky, who created man, who created happiness for man, who made Darius king..."
- Darius the Great, whenever he had the chance to say so

From Darius onward, Achaemenid royal inscriptions generally venerate Ahura Mazda, creator of the universe.  Artaxerxes II in a Susa inscription also mentions Anahita and Mithra, but merely to attribute his personal success.

The Histories document instances of Persian worship of foreign gods, such as the Assyrian Mylitta (Ishtar) and Arabian Alilat (both of whom Herodotus regards as alternate names for Aphrodite), and Apollo.  It's difficult to say whether what we have here is confusion by the Greeks (the Iranians' own Anahita, a female yazata of water and fertility, might be confused with the Semitic goddesses - for his part, Herodotus somehow wound up calling the Persian equivalent "Mitra," who is and always was male and of Indo-Iranian origin), conflation on the Persians' part (Jona Lendering has posited that they saw Apollo as Ahura Mazda, and Anahita did indeed pick up certain attributes from Ishtar), or just worship of new gods distinct from their old ones.

Herodotus futher testified that Persians themselves had no idols, temples or altars, but sacrificed on mountaintops.  The iconic fire temples of modern Zoroastrianism were introduced after the Achaemenid period.  Animal sacrifice continued from pre-Zoroastrian days.

He also claims that Persians cover their dead in wax before burial, while the Magi expose them to scavenging animals and then bury them.  Zoroastrian tradition until very recently was to let dead be completely consumed by vultures, so the rotting flesh won't pollute the sacred earth.  While the Achaemenid kings were often interred in freestanding or cliffside tombs, the use of stone tombs in Iran today stems mainly from urbanization and pressure from the Muslim majority, who found ritual exposure disturbing.  (Indian Zoroastrians may soon go a similar way, as vultures there are almost extinct.)

These facts do not necessarily indicate that Zoroastrianism was not dominant in Achaemenid Persia - for a given value of Zoroastrianism.  The lack of sources from the Persians themselves in this period leaves the distinction between gods and yazatas ambiguous.  The discarding of animal sacrifice has a parallel in Judaism, which also practiced it in this time period but today does not.  The point indeed is not whether ancient Persians were Zoroastrian, but that one cannot look exclusively to modern Zoroastrianism when attempting to describe the religion of the ancients.

Believe it or not, I have seen people do just this, when justifying, for example, the idea that there were no slaves in the Persian empire (there were) or that Jews and Zoroastrians saw each other as fellow monotheists (questionable on several levels).  As when studying the history of any religion, current practice should be seen as a product of development and not a surefire indicator of what that religion was like in the distant past.

Next time:  I will take you shoe-shopping.