Many have objected to the Trinity on the grounds of its mysteriousness. J.R. Illingworth (The Doctrine of the Trinity ) responds that natural theology is no less mysterious.
Natural theology draws from “the material world and the mind of men,” but both “are baffling.” Nature is perplexing, and so is human history: “When we reflect on the long preparation of the earth for man’s inheritance, or the marvellous mechanism of his body, and still more wonderful powers of his mind, we are led to expect great things of him.” But human history hardly displays this greatness, and this suggests “at least in its superficial aspect, a design that has failed” (132-3).
It’s thus a mistake “to suppose that natural theology supplies us with any standard of intelligibility or clearness by which we can test the Christian conception of God to its disadvantage” (134).
Positively, “when confronted with human philosophy, the doctrine of the Trinity assumes a speculative value.” It indicates “the direction in which the solution of some of our most perplexing problems may lie” (136).
For instance: How can we conceive of God as both absolute and personal? “A person is primarily and essentially a self-conscious subject,” and if God is personal He also must be a subject, which “means a subject of experience, one who undergoes experience, or for whom experience exists,” and this necessitates “an object or objects of experience” (136). This isn’t, Illingworth argues, a problem that arose within Trinitarian theology. It is “first discussed by Plato and Aristotle, as pure metaphysicians, long before the Christian era” (138).
What is the object to which God responds so as to be subject of experience? If it is the universe, then God is dependent on the world for His realization, and in that case “His absoluteness vanishes; He ceases to be God.” If the universe is a mode of His being, we are left with Pantheism, and “personality is lost” (137).
If God is absolute person, an “absolute, eternal subject,” then “He must have a correspondingly absolute object, an eternal experience” (137). The Trinitarian solution is “relations in the Godhead, which do not disturb Its absoluteness, because they are internal to Itself” (138).
We can run the same argument with holiness. What would it mean, Illingworth asks, for a monist or unitarian to say that God is holy? Moral life, he says, includes “certain personal or self-regarding virtues,” but “these are rather preliminary to the true moral life, which involves relations with others; - relations of truthfulness, justice, benevolence, service, sympathy, self-sacrificing love” (140). If moral life is social, then “how can we apply moral attributes to God” (141). If God displays His moral character in relation only to creation, then His holiness and righteousness is potential until actualized in relation to creation. But then God is again dependent on creation and no longer absolute.
Illingworth argues that the Trinity offers a resolution: “this doctrine enables us to think of God as, if the term be guarded from any tritheistic connotation, a social being, or society; or, to use what is perhaps safer language, as existing in a mode of which the family, the unit of human society, is the created and faint reflection. And so it becomes possible to conceive of the various relations which constitute righteousness, and especially of love, in which they culminate, as internal to the Godhead; and of holiness, therefore, as the eternal, essential characteristic of God; and the consequence source of that ‘categorical imperative,’ that awful, unqualified, absolute authority with which the moral law addresses the conscience of mankind” (143). In short, “if we are to think of God as personal at all, we must of necessity involve some kind of plurality in the conception” (143).
All this means that the doctrine of the Trinity doesn’t increase “our intellectual perplexities” but “tends rather to their relief” (144).
There is some questionable terminology here (does God “experience”? is Illingworth making too much of a concession to Descartes to define person in terms of “self-consciousness”?). But the overall shape of the argument is sound, and the remarkable thing is that the argument was being put forward in 1907.