This is a response to Francis J. Beckwith's statement and argument that Muslims and Christians believe in the same God.
You can read his original article in the Catholic Thing here: https://www.thecatholicthing.org/2015/12/17/do-muslims-and-christians-worship-the-same-god/
You can listen to the podcast of his talk at a Thomistic Institute event here: https://www.google.com/podcasts?feed=aHR0cDovL2ZlZWRzLnNvdW5kY2xvdWQuY29tL3VzZXJzL3NvdW5kY2xvdWQ6dXNlcnM6NDQ2MzEzNjkvc291bmRzLnJzcw&episode=dGFnOnNvdW5kY2xvdWQsMjAxMDp0cmFja3MvNTU2MTkxNzQ0
Recently, I was listening to the Thomistic Institute podcast where Professor Francis J. Beckwith argued that Christians and Muslims (and also Jews) believed in the same God. He began arguing this because a few years ago, Wheaton College dismissed Dr. Larcyia Hawkins who made the same comment that Muslims and Christians believed in the same God. Wheaton college dismissed her because this went against their Statement of Faith that all faculty members have to sign off on to work there.
Prof. Francis J. Beckwith replied by writing an article in The Catholic Thing defending the belief that Muslims and Christians believe in the same God.
Francis makes a few distinct arguments about why Muslims and Christians believe in the same God which I'll respond to in a Thomistic way since the podcast was from the Thomistic Institute and both Francis and myself both admire St. Thomas Aquinas.
Before I start, I want to say that I have no formal education in philosophy or theology and that these answers are just from my own limited knowledge of Theology and Philosophy. I also want to state that I have nothing against Professor Beckwith or Dr. Hawkins. I've never met either of them and can only say I respect both of them for their academic achievements. That being said, let me respond.
Whether Muslims, Christians and Jews believe in different Gods?
Objection 1: Because Islam, Christianity and Judaism all subscribe to classical Theism, as the belief that there can only be one God, then they must be all believe in the same God. Even though they view God differently, the God they all believe in still has the same properties.
Objection 2: Despite Muslims and Jews not believing God is triune, this still doesn't mean that they believe in other Gods. They are merely disputing a property of the same God.
Objection 3: Abraham and Moses did not believe that God was a trinity. However, we do not argue that they didn't believe in the same God as Christians.
Objection 4: The seemingly contrary beliefs about what or who God is, is merely sensory differences to the same God. The same way the same man may look different to certain people or may even present himself differently to different people is still essentially the same person.
On the Contrary, Aquinas says, "God is absolutely simple, there cannot be false knowledge of him in the sense that something might be known about him and something remain unknown, but only in the sense that knowledge of him is not attained. Accordingly, anyone who believes that God is something that he is not, for example, a body, or something like that, does not adore God but something else, because he does not know him, but something else." (Commentary on the Gospel of St. John, C.4 L.2 603.) the very fact that Christians believe in a Triune God means that Muslims and Jews do not believe in the same God as Christians. God's Triune essence is essential to His being and by rejecting an essential property of God's essence we see Muslims and Jews believe not in the Christian God but false gods.
I answer that, while Jews and Muslims have a similar understanding of what a God is to Christians they lack a key detail in the very being of God. Thus what God is in His very being and essence is different from Christians to Jews and Muslims. To say that Christians and Jews and Muslims believe in the same God is to negate this essential difference. We thus can't believe in the same God since the God we believe in is Triune and the God they believe in is not.
Reply to Objection 1: We do not believe God has the same properties. Christians believe that God is Triune whereas Jews and Muslims do not. If this was any other property that was different such as omniscience, omnipotence, eternal, transcendence, etc then we would say that these are not the same God. Further, Jesus himself, says that Jews do not follow God but follow their own father. (John 8:31-44).
Reply to Objection 2: This different property of God's essence does make it different. Take for example the lower case letter 'p'. If we change one property of this letter then it no longer becomes the same letter. If the letter is flipped vertically then it becomes the lower case letter 'b'. If it is orientated 180º then it becomes the lower case letter 'd'. If it is flipped horizontally then it becomes the letter 'q' and so on. Likewise when we add or subtract an essential property from God then we are dealing with an entirely different entity despite it's perceived similarities.
Reply to Objection 3: Having an incomplete knowledge of a thing is not the same as rejecting knowledge of a thing. I may know of a thing that is a passage way between two areas. I may later find out that it is a door. However, this is different from someone who rejects that it is a door and says instead that it is a gate. There is a clear difference between the man with the incomplete knowledge of the door and the one who rejects that it is a door. Jews can no longer claim to have an incomplete knowledge since Christ has manifested himself and made this new information clear. By rejecting Jesus Christ as God they have made false conclusions about the essential properties of God and thus have a different God to the Christians.
Reply to Objection 4: Sensory differences are dependent only on a thing's accidents and not it's substance. The argument that Muslims, Jews and Christians believe in different Gods is not based on different accidents but on different substances. It doesn't matter whether Jews call their God Yahweh, Muslims their God Allah and Christians their God, Lord God. These are merely accidents. What we have argued is that there are differences in the substance of these different Gods and that the God of the Christians is different to the God of the Jews and Muslisms.
The main problem with Francis's argument is that he confuses God's Triune nature as an accidental property rather than an essential property. While it is possible to disagree about certain accidental properties while still talking about the same substantial being, once you start to disagree with substantial properties then you are bringing the entire being into question.
So, is it possible to argue about the same being but bring into question it's substantial or essential properties?
No. When you argue the essential properties of a being then you are arguing what that being is. If two people are in disagreement about the essential properties of a being then they have in mind two different beings (whether those beings exist or not). Because Muslims and Jews are in disagreement on an essential property of God, it means that they have in mind a different being. Although, I'll admit the being they have in mind is very similar to the being Christians profess as God. The Muslims and Jews unfortunately have a non-existant being in mind when they refer to as their god.
My hope is not to antagonize Francis J. Beckwith but to continue the discussion that he has started. My hope is that after reading this he may be able to respond to the points that I've made and clarify his position a bit better.
God Bless.
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