Simplify the following text for a Grade 9 native English-speaking person.
Original: Zoroastrianism was the dominant religion of Sassanid Persia , and Mazdak himself was a Zoroastrian priest , or mobed , but most of the Zoroastrian clergy regarded his teaching as a heresy .
Simplified: Zoroastrianism was the dominant religion of Sassanid Persia . Mazdak was a Zoroastrian priest .