I am just beginning to rework Katherine Sonderegger’s Systematic Theology, Vol.1, The Doctrine of God for a review I will be writing of it for the journal Cultural Encounters. Right away in the preface something stood out to me, especially in light of all of these posts (from various thinkers) on the issue of Muslims and Christians (and Jews) worshipping the same God. In my last post which I just finished a few hours ago we were taking a look at Bruce McCormack’s response to this whole question. We noticed that he gave the strongest arguments for and against belief that Christians and Muslims worship the same God. In his argument in the affirmative that Muslims and Christians do indeed worship the same God he argued from the principle reality of orthodoxy.
As an illustration of how McCormack’s argument works, the one from orthodoxy, the one that emphasizes the oneness of God (De Deo Uno) as the hook wherein it can be held, ostensibly, that Christians and Muslims worship the same God, Sonderegger writes:
Once more we must pause before a seemingly anodyne, wholly biblical phrase: the One God. Perhaps nothing so marks out the modern in systematic theology as the aversion to the scholastic treatise, De Deo Uno. It belongs not to the preface but rather the body of the dogmatic work to lay out the broad movement in present-day dogmatics that has pressed the treatise De Deo Trino to the fore; indeed, it crowds out and supplants the exposition of the One God. But even here we must say that the doctrine of the Trinity, however central to the Christian mystery, must not be allowed to replace or silence the Oneness of God. God is supremely, gloriously One; surpassingly, uniquely One. Nothing is more fundamental to the Reality of God that this utter Unicity. Such is God’s Nature; such His Person: One. Oneness governs the Divine Perfections: all in the doctrine of God must serve, set forth, and conform to the transcendent Unity of God. Now, to say all this aligns the Christian doctrine of God with the faiths of Abraham, Judaism, and Islam; indeed all of monotheisms—for monotheism is not a shame word! The Christian affirmation of divine Unicity opens it, like the merciful and welcoming Lord it serves, to the peoples and faiths of the good earth. But this cannot serve as ground for such a fundamental axiom in dogmatics. Rather, we must appeal to Holy Scripture.
What Sonderegger writes comports well with what McCormack wrote here:
… The most natural understanding of the oneness of God for those coming directly out of second Temple Judaism was that of “singularity” and/or “uniqueness.” This understanding was given further strength by the influence of Middle Platonism already on the LXX but most especially on the Greek apologists. By the mid-second century at the latest, a concept of God was already firmly in place which owed a great deal to Middle Platonism. The concept in question affirmed that God is one, simple, impassible, invisible, immaterial being. In constructing this concept, there can be little question but that the definition of God’s “oneness” owed a great deal to its “neighoring” concepts, simplicity above all. Unity and simplicity went hand in hand for the early Fathers. And that was one of the reasons (though not the only one) that debates over the doctrine of the Trinity in the fourth century were so difficult and protracted. All of the fourth century theologians whose doctrines of the Trinity would eventually be recognized as orthodox were committed to unity and simplicity (see G.L. Prestige,God in Patristic Thought). And so one of the most important tasks facing fourth century theology was how to think the three-ness of “persons” into or together with an already existing concept of “oneness.”
It is interesting, as a side-note, that Sonderegger is highly antagonistic towards the emphasis that Karl Barth has provided for 20th and 21st century theology with his incorrigible desire to elevate the Triunity of God (De Deo Trino) to the ‘fore’ when thinking God. It is interesting because, relative to McCormack’s own Barth[ian] views Sonderegger is at deep odds with McCormack. McCormack would say that Sonderegger advances the orthodox view the one that allows Christians like Hawkins, Volf, et al. to claim that Christians and Muslims do indeed worship the same God. And as we can see from the Sonderegger quote when Oneness is the dominant fundamentum when developing a doctrine of God, it seems those who take this approach, apparently the orthodox folk, find it compelling at a first-order leveling of things that all monotheisms, at a base level converge at a central point; i.e. simply that God is One. Apparently this is enough common ground for the orthodox to maintain that they worship the same God that the Muslims do.
 Katherine Sonderegger, Systematic Theology, Volume 1, The Doctrine of God (Minneapolis: Fortress Press, 2015), xiv.