Old Persian Texts
The Achaemenian Kings left extensive cuneiform inscriptions in Old Persian dated roughly between 600 BCE and 300 BCE. They also left ruins which have been described as the most grandiose of the ancient world. While it is by no means certain that they were orthodox Zoroastrians, the majority opinion among scholars is that this is very likely. One of the strongest arguments for this is the frequent mention of Ahura Mazda in the inscriptions, which is almost certainly an innovation of Zarathushtra's. Their religion is also described by Herodotus in sufficient detail to leave little doubt that they were basically Zoroastrian.