The Imam is the successor of the Prophet, he is free from all sin and his authority is infallible.  The Imam is the Vicar of God on earth. There have been twelve regular Imams according to Shiah belief.  The last of the twelve Imams, Abu el Kasim, is supposed to be still alive, though hidden from view.  He is the Mahdi, or expected Guide, "who will fill the earth with justice, even though it be covered with tyranny." (Islam, a Challenge to Faith, Zwemer, 1907)