How important was Zoroastrianism to Achaemenid Persians?

The origins of Zoroastrianism are associated with the early Achaemenid Persian Empire. But what is less well known is that Zoroastrianism changed considerably over the Achaemenid period as it was adopted by the royal family and the Persian elite. It was a fluid and adaptive belief system. But it certainly wasn't the only religious belief in Persia. This source-based homework task is designed for students of the NSW ancient History Syllabus for the Australian Curriculum.

Higher School Certificate course
Historical Periods
Option D: Persia - Cyrus II to the death of Darius III

Historical Societies
Option D: Persia in the time of Darius II and Xerxes