10. Zoroastrianism – 2.7 million followers
Zoroastrianism is an ancient religion of Greater Iran. Much like the Roman religion for Rome, it was adopted in differing forms as the generally inclusive overarching state religion of the Achaemenid Empire and subsequent Parthian and Sasanian empires, lending it immense prestige in ancient times. As a result, many aspects of Zoroastrianism, either influenced or were inherited by contemporary and later religious systems, including Second Temple Judaism, Christianity, and Islam. Zoroastrianism was gradually marginalized or otherwise absorbed by Islam from the 7th century onwards with the decline of the Sasanian Empire.