AHC: The 4 great Abrahamic faiths Christianity, Judaism, Islam and...Gnosticism.

Make Gnosticism a powerful religion with millions of adherents and large territories with majority populations. Gnostic theology can be changed to an extent to make it more conversion and nookie happy.
 
Arabs don't conquer the Sassanids but still conquer the Byzantine Levant and Africa. Sassanids are later conquered by Manichaean Turkic tribes creating a Manichaean equivalent to the Seljuk empire. Islamic expansion into India and the Caucasus would be Manichaean expansion instead, while Islam is still a major religion.
 
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