The “do Muslims and Christians worship the same God?” controversy surges on, reported most recently in the Economist. It's worth revisiting.

In an earlier post, I charged that the claim that Muslims and Christians worship the same God (what I will call the “identity theory”) implicitly denies a central tenet of classical theism. That charge needs some filling out, since the departure from classical theism is not the obvious one. Some may say, “There is a class of divine beings, of which both Allah and the Triune God are members.” No orthodox Christian says that. Rather, the argument is that there is only one God, but that Muslims and Christians are worshiping and referring to (however differently) the same God, the sole God who is. 

That argument does imply, it seems, a disruption of the identity of essence and existence in God, one of the key affirmations of classical theism. To see this, we need to unravel the argument in several stages. 

1) According to the identity theory, the only actual referent of the term “God” is the Triune God. He's utterly unique, not a member of any class. This is consistent with classical theism.

2) Identity theory admits, though, that Muslims do not believe that the true God has an eternal Son who became incarnate. That is, Muslims speak about “God” without the specific feature of “having an eternal Son.” According to identity theory, Muslims talk about and worship the true God without believing He is Triune. Identity theorists may say that Muslims are wrong about God's being, but they are wrong about the same being that Christians are right about.

3) For Christians, the qualify of “having an eternal Son” is a defining feature of God. It is not merely a defining feature of the particular God we worship. It is a defining feature of divinity itself. It isn't a supplement to divinity, but constitutes the very essence of deity. To speak of a “God” who does not have this defining feature isn't simply to speak wrongly of the true, Triune God. It is to define the term “God” differently. 

4) Identity theory can make this move only because it assumes the essence of God (as “divine being”) is not identical to His existence (as the specific God who is Father, Son, and Spirit). It defines the divine essence as something other than Triune. No other divine being exists, but “divine Being” becomes a conceptual class within which the Triune God - the God who has a Son - is a specific instance.

5) The affirmation that “God's essence is His existence” is basic to classical theism, since only this preserves the denial that God is metaphysically composite (that is, He is not a sum of any kind, even the sum of essence + existence).

And this yields what might appear a surprising corollary: Far from undermining classical theism, only a theology that makes triunity fundamental to divine being can preserve classical theism. Any other approach - a distinction between the One God and the Triune God - opens up a gap between essence and existence. I suppose Rahner saw this: It was in the interest of preserving classical theism that he wanted to close the gap between the treatise on De Deo Uno and that on De Deo Trino. Only if we say from the outset that the one God is triune can we say that God's essence is His existence.