For this to happen, it would need to become less associated with the Sassanid state. Perhaps the Sassanids survive substantially longer (with the Muslim Arabs being beaten back by the Byzantines early in their attempted expansion and with Abu Bakr being killed perhaps?). Then, a major civil war in the Sassanid Empire leads to the exile of many elites who flee to Central Asia and the Caucasus, where they integrate into the local khanates as refugees. Zoroastrianism eventually establishes itself as the main religious rival to Christianity among the peoples of Central Asia and the Greater Caucasus, as they battle Christianized Slavs. This leads to Zoroastrianism being less associated with just Persia, although the cultural center of the religion remains in the shattered, Arab and Turkic-dominated remnants of Persia. The Zoroastrian khanates eventually unify as they battle pressures to convert. However, Europe, without the pressure of Muslim encroachment, has significantly more variety of Christian sects, with some remaining pagans in the North.