Ahmad ibn Hanbal (d. 241 H/855 AD), renowned for his profound knowledge of hadith--the reports of the Prophet's sayings and deeds--is a major figure in the history of Islam. Ibn Hanbal's piety and austerity made him a folk hero, especially after his principled resistance to the attempts of two Abbasid caliphs to force him to accept rationalist doctrine. His subsequent imprisonment and flogging became one of the most dramatic episodes of medieval Islamic history. Ibn Hanbal's resistance influenced the course of Islamic law, the rise of Sunnism, and the legislative authority of the caliphate....