I spend a fair amount of time in the company of Muslims these days; indeed, at present, by an accident of the seating-chart, I probably spend more time in close propinquity with Muslims, than with Christians, Hindus, Jainists, Jews, and Zoroastrians combined. (Lotta LDS, though. Plus all that could change with the next re-org, as my next podmates might be Zoroastrians.) [Update March 2017: And now, in fact, our branch chief is a Zoroastrian.] There is an effort of good-will on both sides; my Sunni neighbor points eagerly to passages in the Koran, where good things are promised to ‘believers’ (mu’miniin) rather than specifically ‘Muslims’ (muslimiin). A kind-hearted man, he hopes to be with me in Paradise, and not to gaze down on me roasting in Hell. (`Uqbaalak, ya shaykh.)
Now, we Christians know implicitly, that the doctrine of the Trinity is no polytheism:
that Father, Son, and Holy Ghost, are not at all like Apollo, Zeus, and Hera, say, but more like le père Dupont, at once father, and Frenchman, and fireman. But to explain this to our Muslim friends, is difficult. I often have to fall back on saying: the Trinity is a Christian mystery; it may be true or false (in some transcendental sense of those categories), but it is no descendent, direct or indirect, of the sort of pagan polytheism which stuffed the Kaaba with idols, and peopled the trees with dryads, and Olympus with squabbling gods. For us as for you, God Himself -- Allah -- Yahweh -- e’en He -- is indeed One.
Yet this unity by no means necessitates or logically entails, that there could be no parts at variance within the Godhead, even to dissension. (Of course, their absence might be a truth of the Church, and thus beyond dispute; I am speaking here only of logic, the only subject in which I have been ordained.) How indeed could we ever know otherwise (save by some enigmatic Revelation)? After all: We are made in His image, and we ourselves are a bundle of contradictions; and He contains us, as a proper part. (Additionally, that Eloi, eloi passage would seem to point in that direction.)
C. S. Lewis saw rightly when he compared the notion of the Trinity (purely as regards its intellectual coherence, rather than anything theological) with the ‘separate’ faces of a cube -- which latter is, however, nothing more nor less than the sum of all of them. (Indeed, if you were to go with a sort of ‘projective cube’, with antipodal faces identified, our new Cube (topologically a three-torus) would even consist of exactly three parts.)
The historian of physics D’Abro offers a similar parable, along the lines of Abbot’s Flatland (A. D’Abro, The Rise of the New Physics (1939), vol. II, p. 653), his quarry being however, not the Trinity, but the (in some ways similar) “wave-particle duality”: or, as we might term it, the wave-particle identity. He imagines our various perceptions of something we believe to be one entity, but which sometimes seems a triangle, and at others, a circle: eventually we realize that it is a cone, seen now this way, now that. And, regarding the separateness/unity of electricity and magnetism:
The theory of relativity brought about the fusion of the two aspects, no longer by utilizing the background of 3-dimensional space, but by introducing the more refined background of 4-dimensional space-time. The underlying entity, the partial aspects of which are electric and magnetic, were found to be the 4-dimensional electromagnetic tensor situated in space-time.
-- A. D’Abro, The Rise of the New Physics (1939), vol. II, p. 653
The Trinity, we may confide, whatever in its unknowable essence it may be, is at the very least as complex as a tensor.
Pascal (Pensées, 1670 [posthum]), has a very odd passage, asserting the alienating nature of God’s complexity -- or perhaps not -plexity, but monolithicness :
S’il y a un Dieu, il est infiniment incompréhensible, puisque, n’ayant ni partie ni bornes, il n’a nul rapport à nous. Nous sommes donc incapables de connaître ni ce qu’il est, ni s’il est.
Ni s'il est ! -- A useful first step towards an antidote might be to drop that assertion about God's lacking any parts.