Not again! Yes, again. On 5 September 2016 anno domini, in the pages of Crisis Magazine, Fr. Brandon O'Brien opined (emphasis added):
While some similarities may exist between the Christian and Muslim conceptions of God, it is certain that the Christian who prays “Our Father, Who art in Heaven” each day is not praying to the same God as the Muslim who prays “There is no god but Allah and Muhammad is his prophet.” This is because they are not worshipping the same God.
Certain! How's that for theological chutzpah?
The title of the piece is "Why Christians and Muslims Worship Different Gods." The reason is that the Christian and Muslim conceptions of God are drastically different. The doctrine of the Trinity is perhaps the key difference. For normative Christians God is tri-une: one God in three divine persons, Father, Son, and Holy Spirit. It is well-known that normative Muslims reject this trinitarian conception and hold to the radical unity of Allah. God cannot have a son, either in heaven or on earth. This key difference leads to the crucial difference. For Christians, God, or rather God's Son, died on the cross (crux, crucis) for man's salvation, was resurrected, and ascended into heaven body and soul.
So the conceptions of God in the two religions are radically different. But how is it supposed to follow that Christians and Muslims worship numerically different Gods? It doesn't follow! Let me explain.
Suppose Sam's conception of the author of Das Kapital includes the false belief that the author is a Russian while Dave's conception includes the true belief that he is a German. This is consistent with there being one and same philosopher whom they have beliefs about and are referring to. One and the same man, Karl Marx, is such that Sam has a false belief about him while Dave has a true belief about him.
Now suppose Ali's conception of the divine being includes the false belief that said being is non-triune while Peter's conception includes the true belief that God is triune. This is consistent with there being one and same being whom they have beliefs about and are referring to. One and the same god, God, is such that Ali has a false belief about him while Peter has a true belief about him.
What I have just shown is that from the radically different, and indeed inconsistent, God-conceptions one cannot validly infer that (normative) Christians and (normative) Muslims refer to and worship numerically different Gods. For the difference in conceptions is consistent with sameness of referent. So you can see that Fr. O'Brien has made a mistake.
But nota bene: Difference in conceptions is also consistent with a difference in referent. It could be that when a Christian uses 'God' he refers to something while a Muslim refers to nothing when he uses 'Allah.' Consider God and Zeus. Will you say that the Christian and the ancient Greek polytheist worship the same God except that the Greek has false beliefs about their common object of worship, believing as he does that Zeus is a superman who lives on a mountain top, literally hurls thunderbolts, etc.? Or will you say that there is no one God that they worship, that the Christian worships a being that exists while the Greek worships a nonexistent object? And if you say the latter, why not also say the same about God and Allah, namely, that there is no one being that they both worship, that the Christian worships the true God, the God that really exists, whereas Muslims worship a God that does not exist?
In sum, difference in conceptions is logically consistent both with sameness of referent and difference of referent.
Apparently, this is difficult for some to see. My good friend Dale Tuggy writes,
Christians and Muslims disagree about whether God has a Son, right? Then, they’re talking about the same (alleged) being. They may disagree about “who God is” in the sense of what he’s done, what attributes he has, how many “Persons” are in him, and whether Muhammad was really his Messenger, etc. But disagreement assumes one subject-matter – here, one god.
Tuggy is saying in effect that disagreement presupposes, and thus entails, sameness of referent.
I think Tuggy is making a mistake here. Surely disagreement about the properties of a putatively self-same x does not entail that there is in reality one and the same x under discussion, although it is logically consistent with it.
A dispute between me and Ed Feser, say, about whether our mutual acquaintance Tuggy has a son no doubt presupposes, and thus entails, that there is one and the same man whom we are talking about. It would be absurd to maintain that there are two Tuggys, my Tuggy and Ed's, where mine has a son and Ed's does not. It would be absurd for me to say, "I'm talking about the true Tuggy while you, Ed, are talking about a different Tuggy, one that doesn't exist. You are referencing, if not worshipping, a false Tuggy." Why is this absurd? Because we are both acquainted with the man ('in the flesh,' by sense-perception and countless memories) and we are arguing merely over the properties of the one and the same man with whom we are both acquainted. There is simply no question but that he exists and that we are both referring to him. The dispute concerns his attributes.
But of course the situation is different with God. We are not acquainted with God: God, unlike Tuggy, is not given to the senses. Mystical intuition and revelation aside, we are thrown back upon our concepts of God. And so it may be that the dispute over whether God is triune or not is not a dispute that presupposes that there is one subject-matter, but rather a dispute over whether the Christian concept of God (which includes the sub-concept triune) is instantiated or whether the Muslim concept (which does not include the subconcept triune) is instantiated. Note that they cannot both be instantiated by the same item.
The point I am making is a subtle one, and you have to think hard to grasp it. The point is that it is not at all obvious which of the following views is correct:
V1: Christian and Muslim worship the same God, even though one of them must have a false belief about God, whether it be the belief that God is unitarian or the belief that God is trinitarian.
V2: Christian and Muslim worship different Gods precisely because they have mutually exclusive conceptions of God. So it is not that one of them has a false belief about the one God they both worship; it is rather that one of them does not worship the true God at all.
The difference can be put in terms of the difference between heresy and idolatry. If Islam is a Christian heresy, as has been maintained by G. K. Chesterton et al., then the Muslim has false beliefs about the same being about which the Christian has true beliefs. If, on the other hand, the Muslim is an idolator, then he worships a god that does not exist, which obviously cannot be identical to the true God who does exist.
There is no easy way to decide rationally between these two views. We have to delve into the philosophy of language and ask how reference is achieved. How do linguistic expressions attach or apply to extralinguistic entities? How do words grab onto the (extralinguistic) world? In particular, how do nominal expressions work? What makes my utterance of 'Socrates' denote Socrates rather than someone or something else? What makes my use of 'God' (i) have a referent at all and (ii) have the precise referent it has?
For the technical details see the entries collected here.
Most of the writing on this topic is exasperatingly superficial and uninformed, even that by theologians. Fr. O'Brien is a case in point. He thinks the question easily resolved: you simply note the radical difference in the Christian and Muslim God-conceptions and your work is done. Others make the opposite mistake. They think that, of course, Christians and Muslims worship the same God either by making Tuggy's mistake above or by thinking that the considerable overlap in the two conceptions settles the issue.
My thesis is not that the one side is right or that the other side is right. My thesis is that the question is a very difficult one that entangles us in controversial inquiries in the philosophies or mind and language.
You might say it doesn't matter. If Christians and Muslims worship the same God, then Muslims are heretics: they have false beliefs about the true God. If Christians and Muslims worship different Gods, then the Muslims are idolaters: they worship a nonexistent god. Not good either way. This won't be acceptable to Muslims, of course, but why shouldn't a Christian say this and leave it at that?