Alternate History Zoroastrianism As A Major Religion

[mspence]

WI it had managed to become one of the world's major post-Roman religions?

 

[Buba]

How? Easy.
Not lose to Arab Conquest. And become less tolerant - this means Zoroastrianism prevails from Med to Indus and from Arabian Wastes to Siberia and Mongolia.
Effects? Besides history being unreconizeable from OTL from 635AD onward?
No idea.