This revisionist behemoth can be divided into two parts. The first and longer section outlines the characteristics of late medieval Catholicism, its defining ideas and practices. He examines this topic from the perspectives of both laity and clergy, using a variety of sources including, unusually, the material conditions of the churches themselves. If you don't know what a rood screen is and couldn't care less, this part may be slow going for you, but it is necessary to establish the foundations for the point Duffy has been waiting hundreds of pages to make: late medieval Catholicism was vibrant. If that doesn't sound to you like a revolutionary argument that needs to be hidden two thirds of the way through a 700 page text, it is because the book came out almost two decades ago and the revisionist thesis has become a mainstream, although not dominant, historical position.
Before Duffy's groundbreaking work, the accepted explanation for the Reformation was the moribund corruption of the Catholic Church. Protestant historians, who traditionally dominated this subject, presented a picture of late medieval religion in which a hopelessly corrupt church cared nothing for its flock, who in turn resented or disliked the church and were apathetic about its teachings and practices. As soon as an alternative presented itself, the populace happily turned to the new sects to find more active and fulfilling spiritual lives.
In terms of material evidence, this simply wasn't the case. While the Church had plenty of flaws, especially higher in its hierarchy, on the local level it was very active and responsive to the needs of varying communities. Most people attended services with enthusiasm; even if they were not markedly pious, it was the main entertainment available, and priests and architects tried hard to make the experience of church attendance attractive and interesting. Even if congregants were not entirely happy with a particular pastor there is rarely evidence that they were dissatisfied with Catholicism per se. In fact, there is a mass of evidence in the form of wills* that English people had strong views on certain doctrines such as charity, prayer, and Purgatory.
Duffy is clearly not without his own biases, but this study served as an important corrective, and also reemphasized the value of both microstudies and solid, primary source evidence, both of which had been going out of style in favor of more theoretical approaches to history.
*Wills in this period were not mere lists of property but usually included confessions, statements of faith, exhortations to family members, and prayers.