Who is the Mahdī of Muḥammad’s Progeny (Āl Muḥammad) and why are you waiting for his advent?

Question:
Who is the Mahdī of Muḥammad’s Progeny (Āl Muḥammad) and why are you waiting for his advent?

Answer:
Among the issues on which the heavenly religions have consensus of opinion is the “Global Reformer” who shall appear at the end of time. Not only the Muslims but also the Jews and Christians are waiting for the advent of this man who will spread justice in the world. A survey of the Old and New Testaments will make this fact clear. (1)
In this regard, muḥaddithūn narrate that the Holy Prophet (ṣ) has said:
لولم يبق من الدَّهر إلاّ يوم لبعث الله رجلاً من أهل بيتي
يَمْلأها عدلاً كما مُلِئْت جَوْرًا
Even if only one day is to remain in this world, God will surely send a man from my Ahl al-Bayt who shall fill the world with justice and equity just as it has been filled with oppression. (2)
Therefore, as it has been mentioned earlier, the belief in the coming of such a reformer is commonly acknowledged by the authorities of the heavenly religions, and by many traditions regarding the Promised Mahdī presented in the Ṣaḥīḥ and Musnad books of the Ahl as-Sunnah. The muḥaddithūn and scholars of the two Islamic sects (Shī‘ah and Sunnī) have written a good many books regarding him (‘a). (3)
The compendium of these traditions have specified his characteristics and salient features in such a manner that they are exactly applicable to the direct son of Imām Hasan al-‘Askarī (‘a), (66) the 11th Imām followed by the Shī‘ah. According to these traditions, his name is the same as that of the Holy Prophet (ṣ); (67) he is the twelfth Imām, (68) and he is from the progeny of Husayn ibn ‘Alī ibn Abī Ṭālib (‘a). (4)
By Divine Command, the Promised Mahdī was born in 255 AH and he is now living like other people, though in a state of occultation (ghaybah).
It is necessary to note that such a long life is incongruent neither with science and learning, nor with the logic of revelation. Today science is at the threshold of increasing man’s longevity, believing that man has the ability to live for ages and ages, and if one remains safe from defects and vulnerabilities, the prospect for a long life will be brighter. History has also recorded the names of those who had lived to a very great age.
Concerning Prophet Nūḥ (Noah) (‘a), the Glorious Qur’an
Says:﴿فَلَبِث فِيهِم أَلْفَ سَنَةٍ إِلاَّ خَمْسِينَ عَامًا﴾“And he remained with them (his people) for a thousand-less-fifty years.” ( 5)
And regarding Hadrat Yūnus (Jonah) (‘a), it says:﴿فَلَولا أَنَّهُ كَانَ مِنَ الْمُسَبِّحِين٭ لَلَبِثَ فى بَطْنِهِ إِلى يَوْمِ يُبْعَثُونَ﴾“And had he not been one of those who celebrate Allah’s glory, he would have surely remained in its belly till the day they will be resurrected.” (6)
Similarly, from a Qur’anic viewpoint and in the view of all Muslims of the world, Hadrat Khiḍr (‘a) and Hadrat ‘Īsā (Jesus) (‘a) are still alive and they will continue to live?

Refrence:

[1] Psalms of David, Songs 96-97; Book of Daniel, chap. 12 of the Old Testament; Gospel of Matthew, chap. 24; Gospel of Mark, chap. 13; Gospel of Luke, chap. 21 of the New Testament are narrating about the Promised One for whose advent the world is waiting.
[2] Sunan Abū Dāwūd (Egypt: Tāziyyah Press, n.d.), vol. 2, p. 207; Yanābī‘ al-Mawaddah, p. 432; Nūr al-Abṣār, bāb 2, p. 154.
[3] See, for example, Muhammad ibn Yūsuf ibn al-Kanjī ash-Shāfi‘ī, Al-Bayān fī Akhbār Ṣāḥib az-Zamān; ‘Alī ibn Ḥusām ad-Dīn known as Muttaqī al-Hindī, Al-Burhān fī ‘Alāmāt Mahdī Ākhir az-Zamān; Ahmad Amīn Miṣrī, Al-Mahdī wa’l-Mahdawiyyah. The Shī‘ah ‘ulamā’ have written a lot of books in this regard which are too many to enumerate such as Al-Malāḥim wa’l-Fitan, etc.
[4] Yanābī‘al-Mawaddah, part 76, on “virtues”, from Jābir ibn ‘Abd Allāh al-Anṣārī.
[5] Quran, Al-`Ankabut, verse. 14; Ṣaḥīḥ Tirmidhī (New Delhi, 1342 AH), vol. 2, p. 46; Musnad Ahmad ibn Ḥanbal (Egypt, 1313 AH), vol. 1, p. 376.
[6] Quran, As-Saffat, verses, 143-144; Yanābī‘ al-Mawaddah, p. 443.
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Husayni Nasab, S. (translated by:Limba,M, 2007).The shiah rebuts.ABWA (Ahlul-bayt World Assembly) publishing and printing Center.