Christianity, and after the reign of Constantine the Great, sometimes persecuted it. The Sassanid authority clashed with their Armenian subjects in the Battle of Avarayr ( 451), making them officially break with the Roman Church. But the Sassanids tolerated or even sometimes favored the Christianity of the Church of the East. The acceptance of Christianity in Georgia (Caucasian Iberia) saw the Zoroastrian religion there slowly but surely decline, but as late the 5th century it was still widely practised as something like a second established religion. Decline in the Middle Ages Most of the Sasanian Empire was overthrown by the Arabs over the course of 16 years in the 7th century. Although the administration of the state was rapidly Islamicized and subsumed under the Umayyad Caliphate, in the beginning "there was little serious pressure" exerted on newly subjected people to adopt Islam. Because of their sheer numbers, the conquered Zoroastrians had to be treated as dhimmis (despite doubts of the validity of this identification that persisted down the centuries), which made them eligible for protection. Islamic jurists took the stance that only Muslims could be perfectly moral, but "unbelievers might as well be left to their iniquities, so long as these did not vex their overlords." In the main, once the conquest was over and "local terms were agreed on", the Arab governors protected the local populations in exchange for tribute. The Arabs adopted the Sasanian tax-system, both the land-tax levied on land owners and the poll-tax levied on individuals, called jizya, a tax levied on non-Muslims (i.e., the dhimmis). In time, this poll-tax came to be used as a means to humble the non-Muslims, and a number of laws and restrictions evolved to emphasize their inferior status. Under the early orthodox caliphs, as long as the non-Muslims paid their taxes and adhered to the dhimmi laws, administrators were enjoined to leave non-Muslims "in their religion and their land." (Caliph Abu Bakr, qtd. in ). Under Abbasid rule, Muslim Iranians (who by then were in the majority) in many instances showed severe disregard for and mistreated local Zoroastrians. For example, in the 9th century, a deeply venerated cypress tree in Khorasan (which Parthian-era legend supposed had been planted by Zoroaster himself) was felled for the construction of a palace in Baghdad, away. In the 10th century, on the day that a Tower of Silence had been completed at much trouble