The primary instance where the Trinity receives an at-once calculated and yet ambiguous treatment by Tolkien appears in the Ainulindalë’s image of the Flame Imperishable. As the “Creative activity” or power of Ilúvatar that is simultaneously “with” and “within” him and yet “sent forth” from him, the Flame Imperishable is, as Tolkien writes in his commentary on the Athrabeth, “in some sense distinct from” Ilúvatar (MR 335, 345). This comment is of interest as it states that there is indeed the presence of “distinction” or difference within the Creator, while at the same time implying another “sense” in which the Flame Imperishable is in fact not distinct from but is the same as or identical with Ilúvatar, making them together both Eru “the One.” This analysis is dimly indicated in the Athrabeth itself, when at one point in their conversation Andreth tells Finrod about a “rumour” reported amongst those Men of the “Old Hope” that one day “the One will himself enter into Arda, and heal Men and all the Marring [of the world] from the beginning to the end” (321). Andreth, who for her part does not believe the rumor of the Men of the Old Hope, as “all wisdom is against them,” raises the following, reasonable objection: “Eru is One, alone without peer, and He made Eä, and is beyond it; and the Valar are greater than we, but yet no nearer to His majesty … How could Eru enter into the thing that He has made, and than which He is beyond measure greater? Can the singer enter into his tale or the designer into his picture?” (321-2). Finrod replies by reminding Andreth of the simultaneity of Eru’s immanence and transcendence, stating how Eru is in fact “already in it, as well as outside,” to which Andreth agrees but replies that the saying speaks rather of Eru “entering into Arda, and that is a thing wholly different.” When Andreth asks how such a thing could be possible without the Earth—indeed, without created reality itself—being “shattered,” Finrod pleads ignorance, though he does not doubt that, should Eru wish to do this thing, “he would find a way,” but that if “he were to enter in, He must still remain also as He is: the Author without.” In his commentary on this exchange, finally, Tolkien says that in recognizing the possibility of Eru being both “‘outside’ and inside,” Finrod further “glimpses the possibility of complexity or of distinctions in the nature of Eru, which nonetheless leaves Him ‘The One’” (335), something Tolkien further claims to be “actually already glimpsed in the Ainulindalë, in which reference is made to the ‘Flame Imperishable’” (345), not unlike St. Thomas’s discovery, for example, of evidence of the Trinity in the opening words of Genesis. For Tolkien, in other words, Eru’s ability to be simultaneously immanent within while transcendent to his creation, as his metaphysics of eucatastrophe requires, is directly connected with a kind of Trinitarian complexity or distinction within Eru’s own being.
 According to St. Thomas, “In the beginning God created heaven and earth” is to be expounded to mean that God created heaven and earth “in the Son. For as the efficient principle is appropriated to the Father by reason of power, so the exemplary principle is appropriated to the Son by reason of wisdom, in order that, as it is said (Ps. 103:24), Thou hast made all things in wisdom, it may be understood that God made all things in the beginning—that is, in the Son; according to the word of the Apostle (Col. 1. 16), In Him—namely, the Son—were created all things” (ST 1.46.3).