The Holy Prophet Muhammad salullahu alaih wassalam has prophesied about several events that will occur just before the advent of the Day of Judgment. Among these, Rasulullah salullahu alaih wasslam has foretold the advent of one of his descendants, Al Mahdi (the guided one), which will materialize when the believers are severely oppressed in every corner of the world. He will fight the oppressors, unite the Muslims, bring peace and justice to the world, rule over the Arabs, and lead a prayer in Mekkah at which Isa alaih salam (Jesus) will be present.
Throughtout the history of Islam, a few individuals — driven either by the desperate state of Muslims in their community or a selfish pursuit of power and prestige — laid claim to being Mahdi and found a following among the uninformed masses who were looking for salvation from the heavens. For some of these individuals (like Bab of Iran or Mirza Ghulam of India) the claim to being Mahdi was just a stepping stone to the development of heretic sects which broke away from the fold of Islam. Thus, it is at least prudent to visit the facts of this prophecy and avoid falling pray to false claimants that will appear from time to time.
As Muslims, we should remember that the prophecy about Mahdi is one that will come to pass. This prophecy, however, does not absolve the Muslim ummah from its duty to strive in the cause of Allah Almighty, oppose injustice, and seek peace and betterment of human condition. Centuries have passes from the time of the holy Prophet salullahu alaih wassalam and there is a good possibility that many more will expire before the advent of Al Mahdi. Muslims who are negligent in their duty hoping for a savior are committing a grave mistake and are not following the divine decrees ordained in Quran or taught by Rasulullah salullahu alaih wassalam.