By the 3rd century it was already apparent that all attempts to systematize the mystery of the divine Trinity with the theories of Neoplatonic hypostases metaphysics were unsatisfying and led to a series of new conflicts. The high point of these conflicts was the so-called Arian controversy. In his interpretation of the idea of God, Arius sought to maintain a formal understanding of the oneness of God. In defense of that oneness, he was obliged to dispute the sameness of essence of the Son and the Holy Spirit with God the Father, as stressed by other theologians of his day. From the outset, the controversy between both parties took place upon the common basis of the Neoplatonic concept of substance, which was foreign to the New Testament itself. It is no wonder that the continuation of the dispute on the basis of the metaphysics of substance likewise led to concepts that have no foundation in the New Testament—such as the question of the sameness of essence (homoousia) or similarity of essence (homoiousia) of the divine persons.

The basic concern of Arius was and remained disputing the oneness of essence of the Son and the Holy Spirit with God the Father, in order to preserve the oneness of God. The Son, thus, became a “second God, under God the Father”—i.e., he is a divine figure begotten by God. The Son is not himself God, a creature that was willed by God, made like God by divine grace, and sent as a mediator between God and humankind. Arius’s teaching was intended to defend the idea of the oneness of the Christian concept of God against all reproaches that Christianity introduces a new, more sublime form of polytheism.

This attempt to save the oneness of God led, however, to an awkward consequence. For Jesus Christ, as the divine Logos become human, moves thereby to the side of the creatures—i.e., to the side of the created world that needs redemption. How, then, should such a Christ, himself a part of the creation, be able to achieve the redemption of the world? On the whole, the Christian church rejected, as an unhappy attack upon the reality of redemption, such a formal attempt at saving the oneness of God as was undertaken by Arius.

Arius’s main rival was St. Athanasius of Alexandria, for whom the point of departure was not a philosophical-speculative principle but rather the reality of redemption, the certainty of salvation. The redemption of humanity from sin and death is only then guaranteed if Christ is total God and total human being, if the complete essence of God penetrates human nature right into the deepest layer of its carnal corporeality. Only if God in the full meaning of divine essence became human in Jesus Christ is deification of man in terms of overcoming sin and death guaranteed as the resurrection of the flesh. The Athanasian view was accepted at the Council of Nicaea (325) and became orthodox Christian doctrine.

St. Augustine, of decisive importance for the development of the Trinitarian doctrine in Western theology and metaphysics, coupled the doctrine of the Trinity with anthropology. Proceeding from the idea that humans are created by God according to the divine image, he attempted to explain the mystery of the Trinity by uncovering traces of the Trinity in the human personality. He went from analysis of the Trinitarian structure of the simple act of cognition to ascertainment of the Trinitarian structure both of human self-consciousness and of the act of religious contemplation in which people recognize themselves as the image of God.

A second model of Trinitarian doctrine—suspected of heresy from the outset—which had effects not only in theology but also in the social metaphysics of the West as well, emanated from Joachim of Fiore. He understood the course of the history of salvation as the successive realization of the Father, the Son, and the Holy Spirit in three consecutive periods. This interpretation of the Trinity became effective as a “theology of revolution,” inasmuch as it was regarded as the theological justification of the endeavour to accelerate the arrival of the third state of the Holy Spirit through revolutionary initiative.

The final dogmatic formulation of the Trinitarian doctrine in the so-called Athanasian Creed (c. 500), una substantia—tres personae (“one substance—three persons”), reached back to the formulation of Tertullian. In practical terms it meant a compromise in that it held fast to both basic ideas of Christian revelation—the oneness of God and divine self-revelation in the figures of the Father, the Son, and the Holy Spirit—without rationalizing the mystery itself. In the final analysis the point of view thereby remained definitive that the fundamental assumptions of the reality of salvation and redemption are to be retained and not sacrificed to the concern of a rational monotheism.

Characteristically, in all periods of the later history of Christianity, anti-Trinitarian currents emerged when a rationalistic philosophy questioned the role of the Trinity in the history of salvation. The ideas of Arius were revived by many critics, including the so-called anti-Trinitarians of the Italian Renaissance and the humanists of the 16th century. Researchers into the life of Jesus in the 18th century, such as Hermann Reimarus and Carl Bahrdt, who portrayed Jesus as the agent of a secret enlightenment order that had set itself the goal of spreading the religion of reason in the world, were at the same time anti-Trinitarians and pioneers of the radical rationalistic criticism of dogma. The Kantian critique of the proofs of God contributed further to a devaluation of Trinitarian doctrine. In German idealism, Hegel, in the framework of his attempt to raise Christian dogma into the sphere of the conceptual, took the Trinitarian doctrine as the basis for his system of philosophy and, above all, for his interpretation of history as the absolute spirit’s becoming self-conscious. In subsequent theological work, at least in the accusations of some of its critics, the school of dialectical theology in Europe and the United States tended to reduce the doctrine of the Trinity and supplant it with a monochristism—the teaching that the figure of the Son in the life of faith will overshadow the figure of the Father and thus cause it to disappear and that the figure of the Creator and Sustainer of the world will recede behind the figure of the Redeemer.

In a brief but well-publicized episode in the mid-1960s in the United States, a number of celebrated Protestant theologians engaged in cultural criticism observed or announced “the death of God.” The theology of the death of God downplayed any notion of divine transcendence and invested its whole claim to be Christian in its accent on Jesus of Nazareth. Christian dogma was reinterpreted and reduced to norms of human sociality and freedom. Before long, however, the majority of theologians confronted this small school with the demands of classic Christian dogma, which insisted on confronting divine transcendence in any assertions about Jesus Christ.

The transcendence of God has been rediscovered by science and sociology; theology in the closing decades of the 20th century endeavoured to overcome the purely anthropological interpretation of religion and once more to discover anew its transcendent ground. Theology has consequently been confronted with the problem of Trinity in a new form, which, in view of the Christian experience of God as an experience of the presence of the Father, Son, and Holy Spirit, cannot be eliminated.

What it is to be human

The starting point for the Christian understanding of what it is to be human is the recognition that humans are created in the image of God. This idea views God and humans joined with one another through a mysterious connection. God is thought of as incomprehensible and beyond substance; yet God desired to reflect the divine image in one set of creatures and chose humans for this. Man as the image of God belongs, therefore, to the self-revelation of God in quite a decisive way. God, being reflected in the human creature, makes this being a partner in the realization of the divine self; there is constant interaction. Humans find fulfillment in God, the divine prototype, but God also first comes to the fulfillment of the divine essence in relation, in this case, with the human.

The human as a creature

The idea that human beings were created according to the image of God was already being interpreted in a twofold direction in the early church. For one thing, humans, like all other creatures of the universe, are the creation of God, and as creatures human beings stand in a relationship of utter dependency on God. They have nothing from themselves but owe everything, even their being, exclusively to the will of the divine Creator; they are joined with all other fellow creatures through a relationship of solidarity. The idea of the solidarity of all creatures was eventually eclipsed by the idea of the special position of humans and their special commission of dominion. The idea of solidarity with all creatures has been expressed and practiced by but few charismatic personalities in the history of Western piety, such as by Francis of Assisi in his “Canticle of the Sun”: “Praised be Thou, my Lord, with all Thy creatures, especially with our sister sun.”

The second aspect of the idea of the human being as a creature operated very much more emphatically: the superiority of humans over all other creatures. God placed humans in a special relationship to the divine. God created them in the divine image, thereby assigning to humans a special commission vis-à-vis all other creatures.