Complex sentence: For one thousand years Zoroastrianism was one of the world's most powerful religions, in varying forms the state religion of Greater Iran from 600 BCE to 650 CE, including the Achaemenid, Parthian and Sasanian Empires.
Split into simpler sentences: For one thousand years Zoroastrianism was one of the world's most powerful religions. In varying forms Zoroastrianism was the state religion of Greater Iran from 600 BCE to 650 CE, including the Achaemenid, Parthian and Sasanian Empires.