That should be add to a very interesting religious situation before Islamic conquest.
In the eastern provinces, Buddhism, probably a majority of population.
In the northern, stepparian provinces, strong Shamanistic practices below official Zoroastrism.
In the core area, Zoroastrism, with lesser communities of Jews, Christians (later of Nestorian branch) and Manicheists.
In the west, Christianism could be dominant in areas like northern Mesopotamia, but Zoroastrism was strong. Soon, Nestorianism expanded hugely in the empire.