Ali ibn Abi Talib is arguably the single most important spiritual and intellectual authority in Islam after prophet Mohammad. Through his teachings and leadership as fourth caliph, Ali nourished Islam. But Muslims are divided on whether he was supposed to be Mohammad’s political successor—and he continues to be a polarizing figure in Islamic history. Hassan Abbas provides a nuanced, compelling portrait of this towering yet divisive figure and the origins of sectarian division within Islam. Abbas reveals how, after Mohammad, Ali assumed the spiritual mantle of Islam to spearhead the movement that the prophet had led. While Ali’s teachings about wisdom, justice, and selflessness continue to be cherished by both Shia and Sunni Muslims, his pluralist ideas have been buried under sectarian agendas and power politics. Today, Abbas argues, Ali’s legacy and message stands against that of ISIS, Al-Qaeda, and Taliban.