I think the latest realistic POD for Christianity is maybe Clovis concerting to Arianism instead of Catholicism. A Frankish Empire could probably overthrow the Pope or force him to radically depart from the Nicene Christianity previously espoused. After Clovis, though, Catholicism was way too powerful for it to change. Maybe an Arian Christianity makes Islam polytheistic?? Muhammad clearly utilized Christian theology in the formation of Islam, so maybe if Arianism takes hold he makes Islam polytheistic. Other than that, maybe have the Satanic Verses be real and generally accepted as factual doctrine instead of Satan