The Mahdi (“guided one”) is, as Wikipedia puts it, the
the prophesied redeemer of Islam who will stay on Earth for seven, nine or nineteen years- (according to various interpretations) before the Day of Judgment (yawm al-qiyamah / literally, the Day of Resurrection) and, alongside Jesus, will rid the world of wrongdoing, injustice and tyranny.
It’s a basic tenet of Islamic theology, for both Sunni and Shia. The idea of Islam without it would be similar to the idea of Christianity without the apocalypse of the Book of Revelations. And yet that’s exactly what the Ghair-i-Mahdi is.
Quoting the Reverend Hughes’ Dictionary of Islam:
GHAIR-i-MAHDI. Lit. “Without Mahdi.” A small sect who believe that the Imam Mahdi will not reappear. They say that one Saiyid Muhammad of Jeypore was the real Mahdi, the twelfth Imam, and that he has now gone never more to return. They venerate him as highly as they do the Prophet, and consider all other Muslims to be unbelievers. On the night called Lailatu ‘l-Qadr, in the month of Ramazan, they meet and repeat two rakah prayers. After that act of devotion is over, they say: “God is Almighty, Muhammad is our Prophet, the Quar’an and Mahdi are just and true. Imam Mahdi is come and gone. Whoever disbelieves this is an infidel.” They are a very fanatical sect.
The Mahdi has come and gone? How extraordinary dark and gloomy a vision that is. It’d be like a Christian group believing that the Rapture took place centuries ago, and we are living in the world of the left-behind.