Please inform us about the following things:1) What is Mehdi 's (Jamat-e-Mehdiviya) Aqida?2) What should be a Muslim's Aqida about Mehdi ?3) When will Mehdi come? Or has he come and gone?

Praise be to Allah, the Lord of the World; and may His blessings and peace be upon our Prophet Muhammad and upon all his Family and Companions.

I. al-Mahdiyya (the Directed) is a reformative movement that appeared in Sudan. It was established by Muhammad Ahmad al-Mahdi , born in 1260-1302 A.H. corresponding to 1845-1885 A.C. in the Island of at the North of Dunqula city. Here are the main ideas of this movement:
1 - To return to the Qur'an and Sunnah.
2 - To abolish all Sufi groups and to establish a new group under the name of the founder, al-Mahdiyya.
3 - To invite to wage Jihad against colonialism.
4 - The founder claimed that this movement was an order from Allah's Messenger (Sallallahu Alaihi wa Sallam) in a state of wakefulness, not a dream.
5 - He claimed infallibility.
al-Mahdi , as a founder, was influenced by some groups of innovation concerning allegation of al-Mahdiya, while influenced by Muhammad Ibn 'Abdul Wahhab concerning his invitation to return to the Qur'an and Sunnah, Da'wah to Islam and reopening of Ijtihad (making judgment on the basis of Qur'an and Sunnah). In addition, he was influenced by Sufism in relation of adherence to forms of Zikr in some ways and at some times.
al-Mahdi considered the period of time before his coming as al-Jahiliyyah (ignorance in truth and religion) and those who do not follow his ideas as disbelievers.
For more details, consult the book of "al-Mausu'a al-Muyassara fil Adian Wal Mazaheb al-Mu'asira".
II. A Muslim has to believe that Allah, The Great and Almighty, will send al-Mahdi before the Last Day, and that he will govern all peoples with justice after the comprehensive injustice. We also have to believe that al-Mahdi 's name is Muhammad as our Prophet's (Sallallahu Alaihi wa Sallam) name, and al-Mahdi 's father's name as the name of our Prophet's (Sallallahu Alaihi wa Sallam) father's name, and so on, as recorded in al-Sunnah.
For more details about al-Mahdi according to the faith of ahl-al-Sunnah, consult "al-Mahdi: A Truth and myth" by Muhammad Ibn Ismael al-Muqaddam .
Other references include: "La wame' al-Anwar" by al-Safarini", Sharh al-'Aqida al-Tahawiya" by Ibn Abu al-Iz al-Hanafi .
Know that there is contradiction between ahl-al-Sunnah faith concerning al-Mahdi and other groups.
As a final word, nobody knows the time of the coming of al-Mahdi , since such a point is a matter of al-Ghaib (unseen) that only Allah knows. But, for sure, he will come when Muslims are in dire need for one to guide them to wage Jihad against the enemies of Islam. At that time corruption and injustice will prevail on earth.
Allah knows best.