wikishia:Featured Article/2015/52
The Occultation of Imam al-Mahdi refers to the belief that the 12th and the last Shi'a Imam, who is the successor of the Noble Prophet (s), Imam Muhammad b. al-Hasan al-'Askari (aj), also called, "Imam al-Mahdi" was disappeared. The occultation occurred with the permission and will of God and it is one of the basic and mandatory teachings of the Twelver Shi'a. As for the reason behind the occultation, the Hadiths have provided various reasons, testing the Shi'a is one of which.