The belief in an ideal future (in which justice and equity would reign in place of the present injustice and oppression, and which would be ushered in through divine power) is not just an Islamic dogma; it is a natural instinct ingrained in human soul and mind by the Creator. That is why this belief is shared by all religions; it is a common heritage of all who believe in a Creator. The Hindus are awaiting their 10th Avatar (Kalki) to begin the Satya Yuga (era of truth)(1); the Zoroastrians hope for the coming of Saoshyant and the rule of Ahura Mazda(2); the Jews are still waiting for the Messiah, and the Christians look forward to his second coming. Not only the followers of religion, even the pagan Greeks longed for a Utopia, and the confirmed atheists, i.e. the communists, piously teach that a time would come when complete equality and equity would reign on the earth, and the society would be cleansed of oppression and tyranny.
The universal acceptance of this idea and ideal shows that it is a divinely-inspired goal which the humanity is irresistibly proceeding to. What the Islam and the Prophet (PBUH&HP) did was to give us the good news that that era was near at hand, that the awaited divine rule was to be established not in some unknown generation, but specifically by his 12th successor. According to Shiites, that successor was born in the year 256 A.H. in Samarra, Iraq. By this birth, hope culminated into fulfillment, the future became present, the unseen materialized into the seen, and the universal aspiration of a golden age was personified in al-Mahdi, turning the prophecies into a fact.
As such, Shiites believe that he is alive now, yet hidden from the eyes of people, and would reappear whenever Allah wills. It is not that he will be born later, or that he was born and died in the past. The savior in the Hindu, the Jewish, or the Christians religions are not necessarily a present fact; they are generally just an idea. But for Shiites, “al-Mahdi” is not an idea; he is a fact, a reality, a living authority, always in readiness to come out to judge between the oppressor and the oppressed.
Based on Shia teachings, the best deed is to “wait” for the release from adversity (i.e. reappearance of Imam al-Mahdi). Shiites are therefore constantly hoping for the joy and happiness, personified in al-Mahdi, the Imam whom they believe is alive at this very time, and who himself is awaiting the command of Allah to reappear, so that he may strengthen the weak, and judge the oppressors. However, he is hidden from the eyes of people and has not submitted to any unjust ruler, and therefore, he will reappear without having paid allegiance to any oppressor. In fact, besides other characteristics that he possesses, his existence and his whole life is a personification of this "rejection of injustice". Those who believe in him know that they too have to remain steadfast in face of oppression and tyranny; they too have to remain unbending before injustice and inequity.
Nevertheless, according to the Shiites, the responsibility of “al-Mahdi” is not just spreading justice and equity in the world at the end of time, rather he is the successor and remainder of Allah on the earth, and the medium for the bounty of Allah on his creation; he is responsible for guiding mankind, and everyone benefits from his guidance(3). This feeling which may be experienced but not described, creates a spiritual relationship between al-Mahdi and his followers, a psychological rapport between the leader and the masses.
It creates hope in place of despair; it replaces weakness, defeatism and grief with strength, confidence and joy. It gives one the courage to stand up before the tyrants instead of bowing one's head before them in submission. A community which eagerly looks forward to a great future, and whose members are advised to always keep their swords ready and within reach - in order that they could rush to their Imam, at whatever time the call came – can never despair of the calamities, because they believe that the future is for the righteous people(4).
(The above is a selection taken from”Prophecies about occultation of Imam al-Mahdi (AS), by Allamah Sayyid Saeed Akhtar Rizvi (with some changes and addition))
1- In Hinduism, Avatar is a deliberate descent of the Supreme Being to earth. In fact, every manifestation of the God is referred to as Avatar. Hindus believe that Vishnu (the Supreme God of Mercy) has appeared nine times so far, and the tenth appearance is yet to happen. They believe that Kalki Purana or simply Kalki is the tenth manifestation of Vishnu who will appear at a time when the world will be filled with oppression and injustice. He will appear on a big white horse with a sword in his hand to eradicate oppression and injustice, spread justice and equity, and begin the era of truth (Satya Yuga)(for further information, refer to “Introduction to Hinduism” by Simon Weightman or “Encyclopedia of Hinduism” by C. A. Jones and J. D. Ryan)
2- Saoshyant is a figure of Zoroastrianism who brings about the final renovation of the world; he will appear at the end of time and save the world from the devils. He is expected to appear at the end of the last (12th) Millennium, and to bring about the Frashegird (the making wonderful or perfect ending). It will begin with the resurrection of the dead, and reuniting of the soul with body. At that time, the devils will be rendered powerless and destroyed by their enemies. With the permission of Ahura Mazda, Saoshyant will judge between the people (for further information, refer to Zoroastrianism: An Introduction, by Jenny Rose).
3- As mentioned earlier, the sole responsibility of “al-Mahdi” is not to spread justice and equity at the end of time, rather he is the successor and remainder of Allah on the earth; he is the perfect human who is the medium between Allah and not only mankind, but also all forms of creatures. In fact, he has various aspects of existence, one of which being his worldly existence through which the golden age of justice and equity shall be established. Therefore, this promised one is not only alive and existing, but also the existence of all creatures is dependent on him.
4- “And verily we have written in the Pslams, after the Reminder, that My righteous slaves will inherit the earth”, The Holy Quran (21:105).Source: Roshd website.