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The PoD is 640 AD, before the fall of the sassanid empire
First of all, why the third largest religion and not the second? Well, if you can make it be the second largest religion is okay, but I doubt that it could replace hinduism
I idea I have is that the Sassanid empire survives the muslim conquest, and the turks in central asia remains tengrists. later the Turks conquer the sassanids and a new wave of turkish conquerors (alternative Seljuks and Timurids) make their conquests in the middle east and pakistan, estabilishing tengrist realsm in the area, and continue to conquer the byzantine empire and the levant