Creatio ex nihilo praeter deum is not a formulation that can be traced—or not at least one I have traced—to a specific traditional source. I use that phrase in my theology class as a way of expressing, elliptically no doubt, at least something of what C. S. Lewis has in mind when he writes—in his Letters to Malcolm, which students are reading at that point in the class—
“I know that to create is defined as ‘to make out of nothing’, ex nihilo. But I take that to mean ‘not out of any pre-existing material’. It can’t mean that God makes what God has not thought of, or that He gives His creatures any powers or beauties which He Himself does not possess. Why, we think that even human work comes nearest to creation when the maker has ‘got it all out of his own head’” (Letters, Ch. XIV).
I can usually get away with this ellipsis when I’m lecturing undergraduates! This semester, however, four graduate students are also taking the course, and one of them very rightly insisted that I stop and explain myself more precisely. Did this mean, he asked, that God is not only the efficient and final, but also the material, cause of the universe? And, of course, I had to reply in the negative: things obviously aren’t made out of some sort of “God-stuff”. Nevertheless, it is equally obvious that created things have no source but God, so that in some sense they do indeed come forth “from” Him and from “nothing” else.
We need a distinguo, it seems, between senses of “from” or “out of”. In a short chapter entitled “Ex Nihilo, In Deo” (in his Play of Masks) Schuon offers such a distinction, pointing out that
“in the expression creatio ex nihilo, the word nihil determines the meaning of the word ex: in this case ex does not presuppose a substance or container as is normally the case; it simply indicates a possibility in principle, though in this case the possibility is denied precisely by the word nihil in regard to creation, rather as the word ‘with’ indicates a possible object in the expression ‘with nothing’, though in fact what the expression means is ‘without object’”.
I’ve said that things are “obviously” not made out of God in any material sense, but it is also obvious—and indeed a fortiori—that they can’t be made out of some quasi-substantial nothingness. If I had had the presence of mind, I might have told my student that Aristotle’s material cause simply doesn’t apply, except perhaps by way of analogy, when one is considering the causality called creation or the cause called Creator.
As for traditional Christian sources—if not of the Latin formulation itself, then of the essential point it expresses—the following from Philip Sherrard’s Human Image, World Image (p. 154, footnote 10) will perhaps be useful to you:
“Already in the fourth century the Cappadocian theologians recognize that non-being is one of the Names of God. St Gregory of Nyssa, for example, identifies the nihil out of which this world is created with God Himself in His super-essential Non-Being (see H. A. Wolfson, “The Identification of Ex Nihilo with Emanation in Gregory of Nyssa”, reprinted in his Studies in the History of Philosophy and Religion [Cambridge, Mass., 1973]). Creation ex nihilo means God’s own self-creation, His self-manifestation in theophanies, His movement from darkness to light. Nothing (nihil), Eriugena remarks, is another name for God, and creation ex nihilo is creation from or out of God (see Dermot Moran, The Philosophy of John Scottus Eriugena, op. cit., pp. 236 and 238ff).”
Perhaps I should also quote what Sherrard writes in the body of his text before adding this footnote:
“If the phrase ex nihilo is to have any significance beyond that of guarding against the idea that God creates out of some self-subsisting, independent element, the ‘nothing’ in question cannot indicate a purely negative category; on the contrary, it must indicate either the metaphysical Nothing … a world of uncreated spiritual energies in which there is no thing; or, beyond that, beyond even Being itself, a world of pure potentiality, the Ungrund or Abyss in which the unmanifest virtualities or divine Names of God Himself are occluded” (p. 154).
I confess I’m not sure what the distinction is between “metaphysical Nothing” and “Abyss”; but I certainly do agree with his fundamental insight: namely, that everything which exists comes forth, not only from nothing but God, but from the Nothing God is.