Although the Umayyad's were harsh when it came to defeating their Zoroastrian adversaries, claiming responsibility for many of the atrocities towards the Zoroastrian population during warfare, but they did however offer protection and relative religious tolerance to the Zoroastrians who accepted their authority. As a matter of fact, Umar II was reported to have said in one of his letters commanding not to "destroy a synagogue or a church or temple of fire worshippers(meaning the Zoroastrians) as long as they have reconciled with and agreed upon with the Muslims". As a matter of fact Fred Donner says that Zoroastrians in the northern parts of Iran were hardly penetrated by the "believers" winning virtually complete autonomy in-return for tribute-tax or jizyah. As a matter of fact, Donner goes on to say that "... Zoroastrians continued to exist in large numbers in northern and western Iran and elsewhere for centuries after the rise of Islam, and indeed, much of the canon of Zoroastrian religious texts was elaborated and written down during the Islamic period...".