The Reformation transformed Europe, and left an indelible mark on the modern world. It began as an argument about what Christians needed to do to be saved, but rapidly engulfed society in a series of fundamental changes. This Very Short Introduction provides a lively and up-to-date guide to the process. It explains doctrinal debates in a clear and non-technical way, but is equally concerned to demonstrate the effects the Reformation had on politics, society, art, and minorities. Peter Marshall argues that the Reformation was not a solely European phenomenon, but that varieties of faith exported from Europe transformed Christianity into a truly world religion. The complex legacy of the Reformation is also assessed; its religious fervour produced remarkable stories of sanctity and heroism, and some extraordinary artistic achievements, but violence, holy war, and martyrdom were equally its products. A paradox of the Reformation - that it intensified intolerance while establishing pluralism - is one we still wrestle with today.