Skjærvø, Prods Oktor. 2014. Achaemenid Religion. Religion Compass.  8(6), 175-187.

Achaemenid religion” was the religion of the rulers of Iran in the second half of the first millennium BCE and the local form of Zoroastrianism, the ancient religion of the Iranians. The earliest form of Zoroastrianism is known from the Avesta, their sacred texts, which probably originated in the last half of the second and first half of the first millennium BCE, but were transmitted only orally until priests began writing them down in the seventh century. The “Achaemenid religion” is known from cuneiform inscriptions in the local Iranian language, Old Persian, and from tablets in Elamite found at Persepolis, as well as from other sources. It was a dualist religion, postulating the existence of good and evil from the beginning, as well as a polytheistic religion, but with one god, Ahura-Mazdā, outranking the others. Scholarly discussion has centered on the question whether the Achaemenids were real Zoroastrians, in the sense of following the reformed teachings of the historical Zarathustra. As the assumed historicity of Zarathustra and his reform are increasingly being questioned, scholars are now focusing on the interpretation of the inscriptions, notably from the point of view of the orality of Iranian traditions and their relationship with the Avesta, but also increasingly on the editing of the Elamite tablets and mining them for information.