The Messianic idea is not peculiar to the Shí‘ites, but was brought into Islam at an early period by Jewish and Christian converts, and soon established itself as a part of Muḥammadan belief. Traditions ascribed to the Prophet began to circulate, declaring that the approach of the Last Judgment would be heralded by a time of tumult and confusion, by the return of Jesus, who would slay the Antichrist (_Dajjál_), and finally by the coming of the Mahdí, _i.e._, 'the God-guided one,' who would fill the earth with justice even as it was then filled with violence and iniquity. This expectation of a Deliverer descended from the Prophet runs through the whole history of the Shí‘a. As we have seen, their supreme religious chiefs were the Imáms of ‘Alí's House, each of whom transmitted his authority to his successor. In the course of time disputes arose as to the succession. One sect acknowledged only seven legitimate Imáms, while another carried the number to twelve. The last Imám of the 'Seveners' (_al-Sab‘iyya_), who are commonly called Ismá‘ílís, was Muḥammad b. Ismá‘íl, and of the 'Twelvers' (_al-Ithná-‘ashariyya_) Muḥammad b. al-Ḥasan.[405] Both those personages vanished mysteriously about 770 and 870 A.D., and their respective followers, refusing to believe that they were dead, asserted that their Imám had withdrawn himself for a season from mortal sight, but that he would surely return at last as the promised Mahdí. It would take a long while to enumerate all the pretenders and fanatics who have claimed this title.[406] Two of them founded the Fáṭimid and Almohade dynasties, which we shall mention elsewhere, but they generally died on the gibbet or the battle-field. The ideal which they, so to speak, incarnated did not perish with them. Mahdiism, the faith in a divinely appointed revolution which will sweep away the powers of evil and usher in a Golden Age of justice and truth such as the world has never known, is a present and inspiring fact which deserves to be well weighed by those who doubt the possibility of an Islamic Reformation. [Sidenote: Shí‘ite gatherings at Karbalá.]