Karl Rahner’s Major Contribution to the Logical Coherence of the Trinity
Articulated from the Scriptures in contemporary idiom and clarified against a backdrop of a quite severe misunderstanding or, if one likes, heresy, the doctrine of the Holy Trinity is in many ways the life work of the Christian church. Of this the doctrine of the immanent Trinity is a subset, which investigates that which is known, or better, revealed, about the Trinity in its internal relations, as opposed to its interactions with the created order – that is the domain of the doctrine of the economic Trinity.
Orthodoxy concerning the Trinitarian doctrine is preserved through assent to a set of seven pithy statements touching on being and personhood in the Trinity and specifying the relationship between the ousia or the single being or substance of God and the hypostaseis or the three divine persons as well as that which obtains between the distinct persons:
- The Father is God
- The Son is God
- The Holy Spirit is God
- The Father is not the Son
- The Father is not the Holy Spirit
- The Son is not the Holy Spirit
- There is one God.
Any thinker who has attempted to understand and explain the doctrine of the Trinity would have come up against the formidable challenge of showing the logical coherence of a belief that asserts, on the one hand, the numerical solitariness of the deity – that is, that there is only one God, in the sense of a personal being, rather than two or more Gods – and, on the other, the numerical three-ness of something in the Trinity called persons.
From the outset it is apparent, particularly from the well-known phrase coined by the Greek church father Basil of Caesarea – who was one of the three Cappadocian Fathers famed for being the architects of classical Trinitarian doctrine – mia ousia treis hypostaseis, or “one substance, three persons”, that we have here to do with two categories of existence; the one being the substance, and the other, the person.
Theologians have developed numerous ways to comprehend the coherence of the doctrine of the Trinity.
To many, including the Cappadocian Fathers, the paradox is explicable by recourse to the idea of the term ‘God’ as referring primarily to the nature that each of the three persons has in the same way and to the same degree, thus making them qualitatively identical; and to the ensuing different uses of the term ‘God’.
In other words, the term ‘God’ properly refers only to the nature of the three divine persons; that is, to what makes each of them divine. Nonetheless, it is permissible to refer to each divine person individually as ‘God’ in a derivative use of the term. An example in everyday language of such a two-level usage of a concept would be the word ‘man’. In the sentence, “you are just not man enough”, we observe an indication of nature or a set of attributes that is common, hopefully, to men, whereas in “the man took his luggage and left his house”, the same word denotes not an attribute or set thereof but an individual person.
Almost invariably, whenever I had opportunity to present the above view, people have taken issue with that interpretation of the oneness of God, and for good cause: the Bible, particularly in the Old Testament, seems to leave no room whatsoever for any creativity in the understanding of the declaration, “There is one God”, an example being Isaiah 45:5.
Having said that, one should not be too quick to write off traditional conceptions such as just presented, because it is not clear that the fathers were dogmatic about their interpretation and admitted of no other view. With a doctrine like the Trinity, concerning whose mysteriousness and profundity few are in doubt, one can hardly err in keeping one’s conclusions tentative and being ready to consult other viewpoints, especially those which differ markedly from our own. My own study of the writings of the Cappadocian Fathers has revealed such an attitude of humble alacrity toward openness and mutual learning.
In the context of a lack of a perfect solution regarding the logical coherence of the Trinity, the towering German Catholic theologian Karl Rahner (1904-84), one of the most significant Trinitarian thinkers of the last century, makes an important contribution.
Rahner delivered a breakthrough in Trinitarian and Christological theology by insisting on the substantial identity between the economic and the immanent Trinities. His famous dictum, The “economic” Trinity is the “immanent” Trinity and the “immanent” Trinity is the “economic” Trinity, commonly known as Rahner’s Rule, removes the barrier that had been erected between the two doctrines, thereby enabling mutual influence in which the one doctrine enriches the other, the immanent Trinity given relevance by the economic Trinity, and the economic Trinity bestowed eternal value by the immanent Trinity. In the latter case, the biblical record is seen as a revelation of the eternal being of the triune God.
This second movement Rahner avails of in his consequential treatise on the doctrine of the Trinity, The Trinity, by examining the formal nature of God’s interaction with the created order, and establishing the interconnectedness of the various steps of the process. At the risk of over-simplification, one of the analogies Rahner employs is that of the God-given and designed purpose of the human being, and the human being’s fulfillment of that purpose as an inevitable yet voluntary outworking of that design.
With these and other illustrations, Rahner demonstrates, albeit implicitly and without calling attention to his discovery, the way in which, just as the design and will for the human being presupposes the human being’s fulfillment of that purpose and vice versa, within the Trinity, one divine person is intertwined with another.
Although this was by no means a new insight, having already for centuries been encapsulated in the doctrine of perichoresis which asserts that the divine persons inhabit one another in the single being of God, by averring that the incarnation is the proper operation of the Son and no other divine person so that only the Son could have become incarnate and not the Father or the Holy Spirit, suggesting that each divine person has his proper operation or work – though even in this, the other persons are not uninvolved – and quite possibly highlighting the interconnectedness of these proper works, Rahner shows that the divine persons in all their eternity and internal nature and selfness are similarly intertwined.
The crucial finding is as follows: in Rahner’s examples, like the one quoted above concerning the purpose and fulfillment of the purpose of the human being, the steps are really aspects or, better yet, dimensions of the same reality. To use an analogy of mine, this is comparable to a three-dimensional object like a pyramid, and how each side is formed and ‘built upon’ the others. Such an illustration makes it easier to understand what the biblical writers mean when they speak of a divine person as containing another, as the Evangelist John did in his Gospel, citing the words of the Lord Jesus.
In the final analysis, the unity of the Godhead or the Trinity does not consist merely in a relationship of the most sublime intimacy, whereby the divine persons can be likened to the most loving trio of brothers there ever was, to use a human example. It is not, so to speak, an external unity of which we speak when we declare that there is but one God; if unity were only external, we could not speak of how we believe in ‘one God’ but only ‘one divine council’.
Nay, the unity of the Godhead is an internal unity. It is a mutually containing unity in which the one cannot be separate from the other two, and the Lord is able to say that he who has seen Him has in fact also seen the Father, and that the Father is in Him just as He is in the Father. As St Gregory Nazianzen, another one of the Cappadocian Fathers, put it, in the Trinity “there is one mingling of Light, as it were of three suns joined to each other.” (Oration 31.14)
 It is worth noting that these seven statements also guard against three major heresies of the time of the formulation of Trinitarian doctrine: the first three countering Arianism, in which the Son is viewed as being inferior in substance to the Father; the next three, Modalism, in which the Father, the Son, and the Holy Spirit are thought of as being different forms of the same entity; and the seventh, Tritheism, the belief in three Gods.
 For fact-checking and substantiation of this thesis, the conscientious reader is kindly referred to another work by the author of this article: ‘God-ness’, ‘God-ity’, and God: A Historical Study and Synthesis of the Christian Doctrine of the Divine Being (New Orleans, LA: University Press of the South, 2015).
 Trans. J. Donceel (New York, Crossroad, 1997).
 Taken from the translation of a number of orations by Nazianzen, the Nicene and Post-Nicene Fathers, Second Series, rendered by C. G. Browne and J. E. Swallow.