In 1213, Pope Innocent III issued his letter Vineam Domini, thundering against the enemies of Christendom--the "beasts of many kinds that are attempting to destroy the vineyard of the Lord of Sabaoth"--and announcing a General Council of the Latin Church as redress. The Fourth Lateran Council, which convened in 1215, was unprecedented in its scope and impact, and it called for the Fifth Crusade as what its participants hoped would be the final defense of Christendom. For the first time, a collection of extensively annotated and translated documents illustrates the transformation of...