This article records tradition as it has been passed down and reported. Its sources are not yet part of the atlas's verified catalogue.
Zoroastrianism became the de facto religion of the Achaemenid Persian Empire under Artaxerxes I, who ruled from 465 to 424 BCE, more than eight decades after Cyrus the Great had founded the empire. State patronage did not appear at the empire's founding: it took several generations of Achaemenid rule before the Persian court gave the tradition this standing.
The pattern continued under Artaxerxes I's successor. Artaxerxes II ruled for forty five years, from 405 to 358 BCE, and during his reign Zoroastrianism's reach extended from Armenia through Asia Minor and into the Levant, carried by an empire that by then had held the faith as its own for generations.
This period marks the earliest recorded state patronage of Zoroastrianism, more than six centuries before the Sassanid Empire later made it Persia's official state religion outright. The tradition's own founding figure and its earliest following are not independently dated from this period; what the Achaemenid record fixes is only the point at which the imperial state itself took up the faith.
What the reigns of Artaxerxes I and Artaxerxes II changed for Zoroastrianism's spread was the difference between a Persian faith and an imperial one. Once the Achaemenid court patronized the religion, it traveled with the empire's own administration across three continents, carried far beyond wherever it began.