Karl Rahner was one of the major theologians of the twentieth century, who ironically often described himself as a theological amateur. Rahner's career as a teacher took place largely in secular universities (oddly enough, his attempt for a doctorate in philosophy under the auspices of the Jesuits failed; one has to wonder by what standards he was being judged), but he never lost his church-focussed orientation for theology, which had both a magisterial academic tone and a practical liturgical side.
Rahner's work is systematic and vast, so it is somewhat ironic that most of his work concerning the subject of the Trinity (a fairly central concern for most Christians, particularly catholic/orthodox ones) is contained within this small text of barely more than 100 pages. Rahner's concern about then-prevailing ideas of the Trinity was that they had become too disconnected from 'reality', that they bordered upon (if not outright journeyed into) fanciful speculation with no real basis experientially or scripturally. The Trinity is a derivative concept, not explicitly formulated in the scriptures (hence the need for continuing refinement of creeds over the centuries).
Rahner's idea to correct this is deceptively simple -- he states that 'the economic Trinity is the immanent Trinity' and vice versa. What this means is that the God we experience as the Trinity and who impacts our salvation (the 'economic' Trinity) is not a distinct or different entity from the metaphysical being of God (the 'immanent' Trinity). Rahner states that being an incarnationalist, believing that God became human in the form of Jesus, does not in and of itself imply anything about one's trinitarian concept. God is triune in Godself, and what appears to us in three forms or 'persons' of God are in fact not three separate entities or three different manifestations of one God, but extensions of an internal reality of God.
Rahner places great emphasis on the self-communication of God and the self-possession of God. Self-communication is not merely a verbal expression, nor is it always understandable. Ultimately, the Trinity remains an Absolute Mystery, and much of God's self-communication in Father, Son and Holy Spirit remains incomprehensible to us intellectually.
One of the issues of concern for some readers would the be dependence Rahner seems to create on the importance, or indeed necessity, of creation for the Trinity to really exist -- without a creation to whom to communicate, there is some question about Rahner's ontology. However, as God's self-communication can be within the Trinity also, this need not be a concern. This does raise the question of once again separating out the economic and immanent Trinities from each other, and requires more work that was not completed in this text.
Rahner and Barth share a concern for the identification of God's 'real', ontological being and that God we experience in salvific action and history, although they approach things from different starting points. Ultimately, Rahner sees no real separation between the doctrine of the Trinity and the doctrines of grace and Christology. Rahner discusses the difficulties of language -- many options for language have been used over time to try to discuss the different 'persons' of the Trinity, such as 'manners' or 'modes' or 'hypostasic union', and Rahner calls to task the idea that these are easily understood concepts. Of particular difficulty is the idea of 'persons', given modern psychological theories that put so much more meaning onto the term.
This is not simple reading. Rahner, who studied under Heidegger and other philosophy professors, was well schooled and well attuned to the kind of academic philosophical language of that discipline, only rarely departs from what one might call a high-German scholastic language. Even in translation, the text remains weighty. It is worth reading and re-reading, however, to show the intricate manner of Rahner's thought.
This is an excellent edition of Rahner's "The Trinity." Included is an informative introduction by the late Catholic Trinitarian scholar Catherine Mowry Lacugna (to be read both before and after finishing the main book itself). An extremely valuable glossary is also included along with a rather short index of topics. The 6 page glossary is probably worth the price of the book itself.
Rahner maintains that the bulk of religious literature would remain virtually unchanged if the doctrine of the Trinity was deemed false. This slim volume is designed to rectify this situation. Ironically Rahner rarely returns to the subject of the Trinity in his writings after this volume; even his magnum opus "Foundations of Christian Faith" has few references to the doctrine. Regardless, this volume is important as it sounds the trumpet that the Catholic Church believes this doctrine is important and necessary. The 20th Century is replete with theologians from both the Protestant and Orthodox segments of the Christian faith attempting to give prominence to the Trinity. Rahner, one of the Catholic Church's better scholars joins this thrust.
The book is divided into 3 parts. The first deals with the state of the Trinity in Catholic scholarship when Rahner wrote this treatise (1967). Rahner states his method and structure of how he will confront the Trinity with what has become known as Rahner's Rule: "The economic Trinity is the immanent Trinity, and the immanent Trinity is the economic Trinity." This first part has two important aspects. The first is a discussion of whether the Father could have done the part the Son played in salvation and vice versa. This is emphatically denied by referring to the rule stated above. The second discussion worth noting is one which discusses how medieval scholarship (scholasticism) chose to begin a discussion of the Trinity by referring to the unity before it spoke of its multiplicity (One God, then three Persons).
The second section illustrates the doctrine of the Trinity from the viewpoint of the Magisterium. This represents the main lines of the Catholic teachings. This is important for Rahner as he must incorporate his own teachings on the Trinity within this overarching framework and illustrate how it further illuminates the doctrine without overstepping its boundaries.
The third section is Rahner's own synthesis of the doctrine. The best part of this section is Rahner's discussion of the use of the word 'Person' to describe a member of the Godhead. There is talk of switching to more descriptive terms such as 'distinct manner of subsisting' (contrasted with Karl Barth's 'manner of being'). Rahner argues that there is too much individuality (and a sense of duality) read into our use of the word 'Person'. I found this discussion to be the most illuminating of the entire book.
Rahner's dry but informative treatise is important in that it reflects Catholic scholarship on the doctrine of the Trinity. A good follow-up for a current Catholic understanding would be Catherine Mowry Lacugna's excellent 'God for Us.'