Book excerpt: One of the most enduring and influential of all human institutions, the papacy has also been amongst the most controversial. No one who seeks to make sense of modern issues within Christendom -- or, indeed, world history -- can neglect the vital shaping role of the popes. In Keepers of the Keys of Heaven, eminent religion scholar Roger Collins offers a masterful account of the entire arc of papal history -- from the separation of the Greek and Latin churches to the contemporary controversies that threaten the unity of the one billion-strong worldwide Catholic community. A definitive and accessible guide to what is arguably the world's most vaunted office, Keepers of the Keys of Heaven is essential reading for anyone interested in the role of faith in the shaping of our world.