Medievalists, historians, and women's studies specialists will welcome this translation of Herbert Grundmann's classic study of religious movements in the Middle Ages because it provides a much-needed history of medieval religious life--one that lies between the extremes of doctrinal classification and materialistic analysis--and because it represents the first major effort to underline the importance of women in the development of the language and practice of religion in the Middle Ages.
It's always hard to review a classic. Herbert Grundmann's Religious Movements in the Middle Ages was first published in 1935, with a lengthy and updated literature appended on in 1955. A lot of it is outdated: the cohesion of 13th and 14th century "free spirit" heretical groups that's assumed here is heavily questioned in even relatively conservative heresy textbooks (like Malcolm Lambert's Medieval Heresy: Popular Movements from the Gregorian Reform to the Reformation), and the more recent questions about the origin and derivation of heresy raised by historians like R.I. Moore aren't considered. It can be a little dense (you can tell it's a book initially written in German then translated into English) and lots of the conclusions will seem pretty commonplace if you've read much about the 12th or 13th century.
But at the same time, it's kind of groundbreaking. Grundmann was one of the first people to really say a lot of these things that now seem commonplace in textbook on medieval religion, and he was the first to frame heresy in a way that's now standard: as specific responses to contemporary context, rather than as a cohesive collection of dogmas, or as a Marxist rebellion of the lower classes against the powerful. This was a hugely imaginative step for Grundmann to take, particularly since he emerged from the German university system of the early 20th century that tended to emphasize institutional history, politics, and (if you were so inclined) materialist interpretations of history. Instead, he saw the phenomenon of heresy as a social/cultural issue, and he jumps of from there to explore everything from the key differences between Waldes of Lyon and Francis of Assisi to how women's religious movements spawned both German mysticism and the new traditional of vernacular theology. Pretty much every book on any of these subjects that came after him is at least a little in his debt. Broadly speaking, he sees the 12th through 14th centuries as the emergence of new forms of religious life in response to dissatisfaction or efforts at renewal, and then the slow, methodical sorting out of these movements into the orthodox ones that could be effectively pulled under the umbrella of the institutional church and the heretical ones that refused to be accommodated. Francis and the Dominican beguines were among the former, Waldes and Marguerite Porete among the latter. Appropriation and assimilation are the key themes, Innocent III and IV are the key figures doing the assimilating.
While you could read more up-to-date books on heresy, this one is still a really important work. I appreciate how broad-ranging it is (as you can tell by its almost-ridiculously long subtitle of "The Historical Links between Heresy, the Mendicant Orders, and the Women’s Religious Movement in the Twelfth and Thirteenth Century, with the Historical Foundations of German Mysticism"), and how it gives a lot of attention of women's religious movements. Women still get ignored fairly often because they didn't write as much as men (and when they did, it was usually in the vernacular and less preserved), and I'm impressed that Grundmann gives such credence to their aspirations and accomplishments, all the way back in the 1930s.
Grundmann entered the University of Leipzig in 1921, studying first political economics, then philosophy, than literature. Each, he felt, was insufficiently grounded in historical reality. For his doctoral work he choose Joachim of Fiore, a figure concerning whom there was little prior scholarship and primary sources were difficult to obtain. He developed an approach that was rooted in real events but avoided the collection of minutiae that had previously been the hallmark of medieval scholarship. His dissertation and following articles were hailed as "pathbreaking" (although that didn't stop him from being blacklisted by universities over his opposition to the Nazis). Before Grundmann, medievalists basically opted between two branches: doctrinal or materialist. By reinventing religious history as a branch of cultural history, Grundmann created a synthetic approach that examined the life of the mind in the context of political and social history, attempting to capture the complexity of historical reality.
In Religious Movements of the Middle Ages Grundmann examines the common roots of religious movement that grew in different directions and developed different relations with the orthodoxy of the Church. He is especially interested in female religious movements, on which very little work had been done prior. He made the important realization that women's religious practices were significant in the development of vernacular religious literature.
Short version: Grundmann pioneered the historical approach which has since dominated the study of medieval religion.
When a 1933 book still merits an English translation in 1994, you know it's important. Grundmann's exploration of how 13th century medieval men and women alike took religion into their own hands, launching popular religious movement that survive through today, is a foundational text in medieval studies. It's also a model of how to use the different sources available to tell a story, from literature to law to charters.