It was probably about the same period that the grandfather of the great geographer, Ibn Khūrdādbih, was converted through the influence of one of the Barmecides, [705] whose ancestor had been likewise a Magian and high priest of the great Fire Temple of Nawbahār at Balkh. Scanty as these notices of conversion are, they appear to have been voluntary, and the Zoroastrians would seem to have enjoyed on the whole toleration for the exercise of their religion up to the close of the ʻAbbāsid period. With the Mongol invasion a darker period in their history begins, and the miseries which the Persian Muslims themselves suffered seems to have generated in them a spirit of fanatical intolerance which exposed the Zoroastrians at times to cruel sufferings. [706] In the middle of the eighth century, Persia gave birth to a movement that is of interest in the missionary history of Islam, viz. the sect of the Ismāʻīlians. This is not the place to enter into a history of this sect or of the theological position taken up by its followers, or of the social and political factors that lent it strength, but it demands attention here on account of the marvellous missionary organisation whereby it was propagated. The founder of this organisation—which rivals that of the Jesuits for the keen insight into human nature it displays and the consummate skill with which the doctrines of the sect were accommodated to varying capacities and prejudices—was a certain ʻAbd Allāh b. Maymūn, who early in the ninth century infused new life into the Ismāʻīlians. He sent out his missionaries in all directions under various guises, very frequently as ṣūfīs but also as merchants and traders and the like; they were instructed to be all things to all men and to win over different classes of men to allegiance to the grandmaster of their sect, by speaking to each man, as it were, in his own language, and accommodating their teaching to the varying capacities and opinions of their hearers. They captivated the ignorant multitude by the performance of marvels that were taken for miracles and by mysterious utterances that excited their curiosity. To the devout they appeared as models of virtue and religious zeal; to the mystics they revealed the hidden meaning of popular teachings and initiated them into various grades of occultism according to their capacity. Taking advantage of the eager looking-forward to a deliverer that was common to so many faiths of the time, they declared to the Musalmans the approaching advent of the Imām Mahdī, to the Jews that of the Messiah, and to the Christians that of the Comforter, but taught that the aspirations of each could alone be realised in the coming of ʻAlī as the great deliverer. With the Shīʻah, the Ismāʻīlian missionary was to put himself forward as the zealous partisan of all the Shīʻah doctrine, was to dwell upon the cruelty and injustice of the Sunnīs towards ʻAlī and his sons, and liberally abuse the Sunnī Khalīfahs; having thus prepared