(Part I, II)
To turn to Islam, this issue of a redeemer to come at the End of Days, the Mahdi, the rightly guided one, as he is known in Islam, lies at the fault line of the major divide that exists within Islam, that of the Sunnis and the Shi'i. According to Shi'i Islam only the descendents of Ali are allowed to assume the leadership of the Muslim community. They therefore rejected the line of Caliphs from the Umayyad line. In time this political split also came to include matters of religious law and theology as each side developed their own schools of thought.
One might easily think that the Sunni Islam would not have a concept of a Messiah as it has little need little need for a redeemer figure. Unlike Judaism and Shi'i Islam, the Sunnis have no political reasons for promulgating the concept of a redeemer and unlike both Judaism and Christianity Islam as a whole does not have any theological reasons for a redeemer. Islam does not have a concept a Temple nor of the destruction of a Temple. There is no theology of exile and by extension there can be no theology of redemption. Islam does not have animal sacrifices to be restored. Islam does not even have a concept of "Original Sin" for which people need to be redeemed. One is already redeemed through membership in the community. (See Hava Lazarus Yafeh. "Is there a concept of redemption in Islam?" Types of Redemption pg. 168.)The Sunni historical experience has much in common with the Catholic experience; they are both historical victors. Over the course of a single century following the death of Mohammed, Islam went from a religion of tribesmen in Arabia to armies marching into Francia, only to be defeated at the Battle of Tours in 732. The entire Near East and the African side of the Mediterranean basin, the Parthian Empire and much of the former Roman Empire, was now part of the Islamic world. In contrast, at this point there is still no Christian Europe. Nearly a century after this Charlemagne will still be fighting (and forcibly converting) pagans in Northern Germany. The Norsemen to the north and the Slavs to the east are must distinctively, at this point, not Christians. In essence, Sunni Islam has a better claim than even Catholicism for their religion being manifestly true as historical fact for all to see.
Despite all this and despite the fact that the Koran makes no mention of a Mahdi, the concept of a Mahdi, the rightfully guided one, exists even in Sunni Islam even though it is not a central doctrine. According to the fourteenth century Sunni historian, Ibn Khaldun:
Unlike the case of Sunnism, the concept of a Mahdi is crucial for Shi'i. As with Judaism, Shi'i Muslims must contend with the fact that they are a people defined in terms of their being on the losing side of history. They are the faction in Islam that rejected Umayyads, the first Islamic Empire. Their leader, Ali, was assassinated. Ali's son and heir, Husayn, was defeated and killed at the battle of Karbala. The Shi'i managed to eventually to help the Abbasids, who were descended from the family of Mohammad's uncle Hussein, take power, but the Abbasids betrayed them by declaring their support for Sunnism. Shi'i Islam therefore requires a redeemer not only to reverse these political misfortunes, but also to give meaning to their subjugation so that, instead of being a group of outcasts refusing to accept the verdict of history, they can be that small group of the faithful, beloved by God, that kept the faith even when everything seemed to stand against them. The issue of a Mahdi or of an Imam would break Shi'i Islam apart. There were splits in over the fifth and the seventh imams, leading to three major groups of Shi'i. Most Shiites recognized Muhammad al Bakir. There are also Zaydi Shiites who accepted Muhammad's brother, Zayid ibn 'Ali, as the legitimate imam. There was another split after the death of the sixth Imam, Ja'far al-Sadiq (d. 765). The al-Musawiyya followed the Mahdism of al-Sadq's son Musa al-Kazim (d. 799-800). They accepted the succession of eleven Alid descendants as legitimate imams. The last of these imams was Hasan al-Askari (873-78). Twelver Shiism believes that Hasan's son, Mohammad went up to heaven where he dwells as the Hidden Imam, awaiting the End of Days, when he will return to the world.
What should be very clear from this discussion is that there are two sides to Messianism, a counter-political and a side that is very much of this world and its politics. Messianism is a counter to worldly politics by those who have lost the political struggle and an effort to minimize the significance of that lose. This very rejection of worldly politics also, ironically enough, makes Messianism a distinctively political doctrine. It serves to offer a form of worldly politics for those who would otherwise not have it.