In the absence of Islam, which religion becomes the primary competitor to Christianity?

[Masked Grizzly]

Probably Buddhism and Hinduism would end up being the primary competitors to Christianity, with Zoroastrian and to lesser extent Judaism (at least in parts of Arabia/Yemen/etc) likely remaining regional competitors outside of any great leaders playing a similar role as the early Caliphs.

It would be interesting to delve into what other potential religions that do not exist in our world could have arisen.

Maybe one example could be something that combines the forms of Christianity present in OTL Arabia (plus a dash of Marcionism) with the national election of the Arabs (or Romanized Arabs) as chosen via Ishmael (in place of Issac) with his nephew Esau (in place of Jacob) marrying Ishmael's daughter Mahalath to sire an alternate 12 tribes?

 

[GenghisKhanfan]

Zoroastrianism would be interesting, but Buddhism more likely.

I could see a revival of Zoroastrianism, perhaps with some Buddhist influences, starting in Persia and possibly spreading into the rest of the Middle East, the Balkans, and Arabia. If there were any vestigial remnants of Islam in Arabia, its likely that it would be submerged into Zoroastrian doctrine...

Think about it, A Zoroastrianism which abosrbs what might remain of Islamic-type doctrine, with Buddhist influences, spreading into the Balkans and possibly even merging with a revived Greek Olympian theology. Ahuru Mazda (Zoroastrian) under Buddhist influence, merged with the Greek Olympian god Zeus

 

[Wendell]

I could see a revival of Zoroastrianism, perhaps with some Buddhist influences, starting in Persia and possibly spreading into the rest of the Middle East, the Balkans, and Arabia. If there were any vestigial remnants of Islam in Arabia, its likely that it would be submerged into Zoroastrian doctrine...

Think about it, A Zoroastrianism which abosrbs what might remain of Islamic-type doctrine, with Buddhist influences, spreading into the Balkans and possibly even merging with a revived Greek Olympian theology. Ahuru Mazda (Zoroastrian) under Buddhist influence, merged with the Greek Olympian god Zeus

I don't see it supplanting Christianity in the Balkans. Interesting ideas though.