Anyone familiar with the gargantuan theological output of the German Jesuit theologian Karl Rahner (1904–1984) can hardly miss the irony that for someone who is universally hailed as the most influential contributor to the renaissance of trinitarian theology in the twentieth century, at least in the Catholic Church, and who vigorously insists that the Trinity be the center of theology and Christian life, the bulk of his explicit writings on the Trinity is minuscule. His synthetic magnum opus Grundkurs des Glauben (Foundations of Christian Faith) contains barely four pages on the Trinity, entitled “Towards an Understanding of the Doctrine of the Trinity.” Apart from a handful of pieces in his Schriften zur Theologie (Theological Investigations) and two entries in the encyclopedia Sacramentum mundi, Rahner's longest writing on the Trinity is a booklet-length contribution to a handbook of theology.

Why, then, in spite of the paucity of his writings on the Trinity, is Rahner celebrated as the initiator of the rediscovery of the Trinity in Catholic theology? Does this paucity reflect a lack of consistency between Rahner's theory and praxis? Or could it be argued that it is precisely because the Trinity so thoroughly informs the structure and contents of Foundations of Christian Faith that a lengthy treatment of it is unnecessary? What is so significant about his trinitarian theology, and in which ways has it renewed the Christian theology of the Trinity? To answer these questions would require, first, situating Rahner's trinitarian theology in the context of the Catholic neo-scholastic or manualistic theology which was the staple fare in Catholic seminaries until the 1960s and, second, examining in detail, albeit within a very limited space, its major tenets.

“All authority in heaven and on earth has been given to me. Go therefore and make disciples of all nations, baptizing them in the name of the Father and of the Son and of the Holy Spirit” (Mt 28:18–19, NRSV). So is Jesus reported to have said to his eleven disciples on a mountain in Galilee. While biblical scholars dispute whether these words are Jesus’ ipsissima verba or a baptismal formula of the early church retroactively placed on Jesus’ lips, the verse is an incontrovertible indication that faith in God who is Father, Son, and Holy Spirit, in whose name (note the singular “name” and not “names”) baptism is administered, is already present in the New Testament itself. It has been correctly pointed out that the Christian faith in the Trinity should not be understood to be based exclusively on explicitly triadic formulae such as the above-cited verse, 1 Corinthians 12:4–6, 2 Corinthians 13:14, 1 Peter 1:2, and so on. Rather, the trinitarian data of the New Testament include all the exceedingly numerous texts that speak of the relationship between Jesus and the Father, between Jesus and the Spirit, between the Father and the Spirit, and among the Father, Jesus, and the Spirit. Indeed, the literary structure itself of most New Testament books is arguably trinitarian. In addition, the reality of the Trinity is present not only in certain New Testament formulations but also in the events of Jesus’ life and ministry, in particular his conception, baptism, transfiguration, and death and resurrection, and at the Pentecost. Finally, it can reasonably be claimed that there are already intimations or adumbrations of the Trinity in the Old Testament such as the many names used for God (e.g., Wisdom, Word, Spirit), the “angel of Yahweh” figure, and some theophanies (e.g., the three men in Gen 18:1–2 or the threefold Sanctus of Isaiah's vision in Isa 6:3).

About half a century ago, when the two arguably greatest theologians of the twentieth century, Karl Barth and Karl Rahner, first wrote on the Trinity, it was de rigueur to bemoan the marginalization of the Trinity from theology and spirituality and the dearth of works on the subject. Today writing on the Trinity has become something of a cottage industry, and the trinitarian mystery is unquestionably enthroned at the heart of Christian theology. But one of the drawbacks of the recent proliferation of trinitarian treatises is that with the rise of an enormous plethora of issues and views concerning the Trinity there is the danger of missing the forest for the trees. To obviate this pitfall, the following reflections are offered, not as a bibliographical survey but as a theological map to help readers identify the main issues, tenets, and directions in contemporary trinitarian theology. Writing in 1993, Ted Peters identified twelve issues in contemporary trinitarian theology and various responses to them. Since then other issues and answers have emerged. In a recent work Gerald O'Collins has also identified twelve issues in contemporary trinitarian theology, though his list is slightly different from that of Peters. I will divide my own list into three categories: methodology, doctrine, and practice.

The subject matter of this book is the Trinity, now widely acknowledged to be a mystery of salvation and the central Christian belief. That the Triune God is so now may be a commonplace, but that has not always been the case. One of the pleasant surprises in contemporary theology is the widespread revival of the doctrine of the Trinity. Long shunted to the wings, the Trinity is now occupying center stage. Karl Rahner's oft-quoted bon mot that Christians are, theologically speaking, “mere monotheists” (that is, unitarians) may have been true in the 1960s and earlier. Fortunately, it is no longer so. Recently, a spate of books and articles on the Trinity by Catholic, Orthodox, and Protestant theologians has restored this neglected Christian doctrine to its rightful place. These works have not only retrieved the classical teachings on the Trinity through serious historical research, but have also given the lie to the claim that the trinitarian dogma is nothing more than abstruse metaphysics and a conundrum of “higher mathematics” of one-equals-three and vice versa. They have shown how trinitarian theology is necessary for a full understanding of such burning issues as the nature of the human person, suffering, sexism, ecology, social and economic justice, interreligious dialogue, and so on.