The situation where conversion to Islam was penalized in an Islamic state could not last, and the devout Umayyad caliph Umar II (717–720) has been credited with changing the taxation system. Modern historians doubt this account, although details of the transition to the system of taxation elaborated by Abbasid-era jurists are still unclear. Umar II ordered governors to cease collection of taxes from Muslim converts, but his successors obstructed this policy and some governors sought to stem the tide of conversions by introducing additional requirements such as circumcision and the ability to recite passages from the Quran. Taxation-related grievances of non-Arab Muslims contributed to the opposition movements which resulted in the Abbasid revolution. Under the new system that was eventually established, kharaj came to be regarded as a tax levied on the land, regardless of the taxpayer's religion. The poll-tax was no longer levied on Muslims, but the treasury did not necessarily suffer and converts did not gain as a result, since they had to pay zakat, which was probably instituted as a compulsory tax on Muslims around 730. The terminology became specialized during the Abbasid era, so that "kharaj" no longer meant anything more than land tax, while the term "jizya" was restricted to the poll-tax on dhimmis.