In this volume, the eminent theologian Joseph Ratzinger, now Pope Benedict XVI, collects the work developed in recent decades, to set a whole that develops systematically questions the structure of Christianity. With this vision, are discussed in the first place the relations between the structures, content and skills to bear in mind the context of the doctrine of principles. Below, we discuss the formal principles of Catholicism, first in themselves, this is essentially writing and coordinating the relationship between tradition and faith, history and metaphysics. The second part of the book puts these thoughts in an ecumenical context. It delves into the subject of this tradition, which becomes the basic problem of ecumenical controversy, namely the problem of concrete personal form of tradition in the sacrament of apostolic succession. From here comes the task of analyzing in what sense the "catholicity" is a formal structure of Christianity. One third of all oriented to structural problems of theology and reaches, at the same time, through his anthropological situation, the inflection begins to return to the objective problems were the starting point in the approach of the work. After considerable back reflections to the true fundamental theology in recent years, this book engages in a comprehensive dialogue between different theological positions, Catholic, Protestant and Orthodox, and carries the study of specific issues to their basic supports deeper to discover and, at the same time, ways of genuine unity and understanding. The set of materials collected by Joseph Ratzinger structure here is the foundation on which to build a strong body of doctrine.