The Abbasid caliph.
Useful information: al-Ma'mun, the Abbasid caliph from 198/813 to 218/833, had Mu'tazilite leanings and since in his religious views he favored intellectual demonstration, he was more inclined to give complete freedom to the discussion and propagation of different religious views. Shi'ite theologians and scholars took full advantage of this freedom and did their utmost to further scholarly activities and propagate Shi'ite teachings. Also, al-Ma'mun, following demands of the political forces at the time, had made the eight Shi'ite Imam his successor, as is recounted in most standard histories.