The religion's priests, successors to the pre-Zoroastrian Magi, acquired great power by their command of the techniques of purification. The priests also had great influence on the government in the first period of Zoroastrianism, that under the Achaemenids, when it was for a time the state religion. Alexander's conquest of Persia and the collapse of the Achaemenids destroyed the privileged position of Zoroastrianism. Little is known of the religion for the next 500 years, except that an offshoot, Mithraism (stemming from the worship of Mithra), was taking hold farther west. Zoroastrianism reemerged (c.A.D. 226) under Ardashir I, who established the Sassanid dynasty and fostered a general revival of Achaemenian culture. For four centuries Zoroastrianism was the state religion of the Sassanids, and it successfully met the challenge of nascent Christianity and, later, of heretical Manichaeism. In the mid-7th cent. Persia fell to Islam, and Zoroastrianism largely disappeared. The Parsis of India, centered on Mumbai, probably form the largest group of modern Zoroastrians, who are estimated to number between 124,000 and 190,000. Estimates of the number of persons (concentrated in Yazd, Tehran, and Kerman) who practice the religion in Iran today vary widely. Zoroastrianism affected Judaism (particularly during the time of the Captivity) and, through Gnosticism, Christianity.