Hussain bin Ahmad or Abu Abdullah, surnamed az-Zaki, known as Hussain ar-Radi, or Radi Abdullah (Servant of God who is satisfied and content), was born in 210/825 and assumed the Imamate in 225/840. He is also called Muhammad and al-Muqtada al-Hadi. His also kept his identity secret being represented by his hujjat, Ahmad, surnamed al-Hakim.
Tabari (3rd vol., p. 2232) refers to his son, al-Mahdi under the name of Ibn al-Basri (the son of Basra), emphasising the connection of Radi Abdullah with southern Mesopotamia and the adjoining province of Khuzistan.
The Abbasid caliph al-Mutasim (218-227/833-842) was followed in succession by al-Wasik (227-232/842-847), al-Mutawakkil (232-247/847- 861), al-Muntasir (247-248/861-862), al-Mustain (248-252/862-866), al-Mutaz (252-255/866-869), al-Muhtadi (255-256/869-870) and al- Mutamid (256-279/870-892).
Radi Abdullah is celebrated in devoting time to complete the task of his father, his teachings and institutions. In his time, the faith of the Ismailis spread by leaps and bounds with galloping speed through out the length and breath of Arabia.
Radi Abdullah was an erudite scholar and is celebrated to have epitomised "Ikhwan as-Safa" into an instructive synopsis (al-jamia). Its full name was "ar-Risalat al-Jamia" (the comprehensive espistle). It served as a substitute for the Epistle of "Ikhwan as-Safa" and was intended for private circulation among the more advanced members of the groups. The al-Jamia is the backbone of the Epistles, which was further summarized in "Risalat al-Jamiat al-Jamia an al-Zubdah min Rasail Ikhwan as-Safa" (the condensation of the comprehensive epistles, or the cream of the epistles of Ikhwan as-Safa).
It must be known that the monograph of "ar-Risalat al-Jamia" was awarded the first Howard Bliss Prize by the American University of Beirut in 1929, and was subsequently published serially in the Journal of that institution, vide "al-Kulliyat" (vol. xvii, 1930-1).