However, Sir Thomas Walker Arnold doubts the entire narrative of the forced conversions of the Zoroastrians, citing many examples of tolerance that were shown by the Muslim overlords concluding that "in the face of such facts, it is surely impossible to attribute the decay of Zoroastrianism entirely to violent conversions made by the Muslim conquerors". Arnold suggests that some of the conversions of the former-Zoroastrians were actually sincere citing the similarities between the two religions as a motivation for the conversions. Stepaniants also (like Arnold) declares that some historians have said that some of the conversions to Islam were sincere citing the fact that Islam offered a broader door of brotherhood, unlike the restrictive criteria of Zoroastrianism. Nevertheless, Sir Thomas Arnold does acknowledge that the persecution of Zoroastrians did take place later on. Stepaniants states that many persecutions took place during the reign of the Abbasids, and around that time was when the Parsi exodus took place. But regardless, both Arnold and Stepaniants say that the Islam is not to blamed entirely for the decline of Zoroastrianism. Furthermore, the population of the city of Nishapur, even after the event of conquest (despite conversions to Islam taking place almost immediately) there still remained sizeable Zoroastrian populations, along with the Jews and Nestorian Christians as well. Fred Donner says that the northern were hardly penetrated by the "believers" for a century or the, the Iranian nobility who reside in that area made terms with the believers winning virtually complete autonomy over the region in return of a tribute-tax or jizyah. Donner also acknowledges that Zoroastrians continued to exist in large numbers even after the rise of Islam in these regions.