Although the Umayyad's were harsh when it came to defeating their Zoroastrian adversaries, they did 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". 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. Donner adds "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."