Ismāʿīlism (Arabic: الإسماعيلية‎ al-Ismāʿīliyya; Persian: اسماعیلیان‎; Sindhi: اسماعيلي‎; Esmāʿīliyān) is a branch of Shia Islam. The Ismāʿīlī get their name from their acceptance of Imam Isma'il ibn Jafar as the appointed spiritual successor (Imām) to Ja'far al-Sadiq, wherein they differ from the Twelvers who accept Musa al-Kadhim, younger brother of Isma'il, as the true Imām. Ismailism rose at one point to become the largest branch of Shī‘ism, climaxing as a political power with the Fatimid Caliphate in the tenth through twelfth centuries. Ismailis believe in the oneness of God, as well as the closing of divine revelation with Muhammad, whom they see as "the final Prophet and Messenger of God to all humanity".