The Prophet said: The Mahdi will be of my family, of the descendants of Fatimah [Muhammad’s daughter]. Throughout the Islamic world today there is a call for the restoration of the Islamic Caliphate. The Caliph (Khalifa) in Islam may be viewed somewhat as the Pope of the Muslims. The Caliph is viewed as the Vice-regent for Allah on the earth. It is important to understand that when Muslims call for the restoration of the Caliphate, it is ultimately the Mahdi that they are calling for. For the Mahdi is the awaited final Caliph of Islam. As such, Muslims everywhere will be obligated to follow the Mahdi.
If you see him, go and give him your allegiance, even if you have to crawl over ice, because he is the Vice-regent (Khalifa) of Allah, the Mahdi. He will pave the way for and establish the government of the family [or community] of Muhammad… Every believer will be obligated to support him. The Mahdi is believed to be a future Muslim world leader who will not only rule over the Islamic world, but also the non-Muslim world as well.
The Mahdi is said to lead a world revolution that will establish a new Islamic world order throughout the entire earth: The Mahdi will establish right and justice in the world and eliminate evil and corruption. He will fight against the enemies of the Muslims who would be victorious. He will reappear on the appointed day, and then he will fight against the forces of evil, lead a world revolution and set up a new world order based on justice, righteousness and virtue…ultimately the righteous will take the world administration in their hands and Islam will be victorious over all the religions.