Zoroastrianism was once one of the most prominent religions in the world. Once Iran was conquered by the Muslims, however, it began to decline. Now there are fewer than 200,000 Zoroastrians left, though it's growing more popular in Iraqi Kurdistan, so we may see them significantly expand in numbers.

Your challenge is to keep Zoroastrianism as a major religion. It doesn't necessarily have to be as powerful as it was before the rise of Islam, but there should be at least as many Zoroastrians as there are Jews.
 
Well assuming that the Muslim invasions still occur at the same time as OTL, any POD that would keep Zoroastrianism a major religion on the world stage would have to occur before then or at least before Islam becomes the majority faith in the area.

This means making Zoroastrianism more popular or palatable to the ordinary people instead of Christianity, Mazdakism, or Manichaeism during late antiquity. Probably spread missionaries to places beyond Persia like Central Asia, India, maybe China. You could also strengthen the Sassanid empire by removing the last Byzantine-Sassanid War entirely thus leaving both empires strong enough to face the Muslims. That or set up a Persian-Zoroastrian rebellion during a weak point in the Caliphates.

Great, now I’m thinking about this and it’s going to consume my brain for a long while.
 
Zoroastrianism is not much of a faith that proselytizes so it’s not gonna actively convert people the same way Christianity does.

Avoiding the last Sassanian/Roman War would help out a lot though I am wonder if maybe a deal would be made that would have Iran and Rome have “spheres of influence.” Like one side of the world is theirs to influence and vice versa.

Zoroastrianism could then partially fill the void in the East by spreading down East Africa and also across Asia though I would not see much luck except in less established states.

Alternately, they end up going north through Central Asia and the Caucasian gates and the Slavs convert to Zoroastrianism because of people seeking shelters in the fire temples during winter
 
We could always have it change to a faith that may proselytize others and spread it to the Turks/Mongols in place of Manichaeism so that way it survives beyond Persia. It depends on the POD of course and if that were that case, then it would have to be somebody else besides the Sassanids who gain power. No offense to the Sassanids but I've read that they supported a more Zurvanite version of Zoroastrianism zealously, which is why other denominations and religions gained a foothold in Persia during late antiquity. The Parthians were far more loose on the subject of religion by comparison so maybe a continuation of the Parthian Empire of sorts or another Persian dynasty with different views on Zoroastrianism rebels instead of the House of Sasan.
 
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