God, the Cause of all, is one. This does not mean one as in one of a pair, nor one like a species (which encompasses many individuals), nor one as in an object that is made up of many elements, nor as a single simple object that is infinitely divisible. Rather, God is a unity unlike any other possible unity. This is referred to in the Torah (Deuteronomy 6:4): “Hear Israel, the Lord is our God, the Lord is one.”
Maimonides, from his 13 principles of faith.
“The three words ‘El,’ ‘Elohim,’ and ‘Yhwh’ connote one and the same person, as one might say, ‘King, Emperor, Augustus'”
Talmudic Rabbi Simlai, Yer. Berakhot 9:12d, a Rabbi who in fact argued against Origen on the Trinity.
“if a man claims to be God, he is a liar.”
Talmud Yer. Ta’anit 2:1
All three of those quotes are from highly authoritative, post-Christian Rabbinic Jewish authorities. The debacle at Wheaton College, (an evangelical University), which was over a political science professor claiming that the God of the Christians and God of the Muslims is, in fact, the same God, has led to an ongoing debate on the subject, whether or not the God of Islam and the God of Christianity is in fact the same God.
Now I myself tend towards the position that they are the same God, but I’m not dogmatic about it because the question is extremely complicated and philosophical (if you want to see how sticky it can get listen to a dialogue between Philosophers Dale Tuggy and William Vallicella here and here). However, what I will say is that if one holds to the position that the God of Islam and the God of Christianity are not the same God while at the same time claiming that the God of Christianity is the same God as the God of Judaism then your position is fallacious. This is especially the case when the main thrust of your argument revolves around the Trinity.
One of the people very vocal on the “different god” side is Nabeel Qureshi, someone who I have written about before (here and here). He defends his position in 2 debates (that I’ve heard), one against Miroslav Volf, and one against Joseph Cumming as well as in this article. Now when it comes to the question of why one would say that Jews worship the same God as Christians but Muslims do not, this is what Nabeel Qureshi says:
The response should be obvious to those who have studied the three Abrahamic faiths: the Trinity is an elaboration of Jewish theology, not a rejection. By contrast, Tawhid is a categorical rejection of the Trinity, Jesus’ deity, and the Fatherhood of God, doctrines that are grounded in the pages of the New Testament and firmly established centuries before the advent of Islam. Most of the earliest Christians were Jews, incorporating their encounter with Jesus into their Jewish theology. Nothing of the sort is true of Muhammad, who was neither a Jew nor a Christian. Islam did not elaborate on the Trinity but rejected and replaced it.
Additionally, Volf’s assumption that Jews did not worship something like the Trinity is unsubstantiated. Many Jews held their monotheism in tension with a belief in multiple divine persons. Though the term “Trinity” was coined in the second century, the underlying principles of this doctrine were hammered out on the anvil of pre-Christian Jewish belief. It was not until later, when Jews and Christians parted ways, that Jews insisted on a monadic God. The charge of Christian hypocrisy is anachronistic.
Now this is a little bit of a trick. The Trinity didn’t exist prior to its development centuries after Jesus’ death, so the question of whether or not God was an individual being inhabited by multiple instances, or persons wasn’t a question, God was an individual person (we’ll get to his “underlying principles being hammered out in pre-Christian Jewish belief” statement later). Now I want to approach this by analogy, was Adam Smith an anti-Marxist? Well, no, Karl Marx wasn’t born when Adam Smith was alive and thus Adam smith could have no opinion on Marxism because the ideology didn’t exist, but that doesn’t mean that the two theories are in conflict. The followers of Adam Smith tend to be against Marx and read the theories of Marx as contrary to those of Smith, even though Marx himself thought himself to be part of the Political Economy tradition, much of what he wrote was based on Smith, accepting Smith (and Ricardo’s) categories and building on and critiquing them.
However, when we talk about Judaism, we’re talking about Judaism, as we know it today, the mainstream rabbinic Judaism and their predecessors. Just as it would be silly for Muslim scholars to say that Christians believe in their conception of God and cite Arians, Ebionites, Nazarenes and so on, as it is for Nabeel Qureshi to claim Jews worship the same God as Christians on the basis on a supposed Jewish trend for a multi-personal Yahweh (which in fact is a fantasy). The Jewish God is a firmly Unitarian God, Jews from the beginning, when they were addressed with the idea of an incarnate God or a multiple person God rejected that idea.
So if Nabeel is going to say that Jews (and that means Jews living today) and Christians worship the same God on the basis of some supposed Jewish trend toward a multi-personal God, which doesn’t exist anymore (or actually ever really), one would have to say that Muslims are perfectly in their right to say Christians and Muslims have the same conception of God based on the Ebionites, Nazarenes, Arians and other historic Christian Unitarians. Of course, both propositions are wrong.
Now Nabeel has, along with other Trinitarian apologists, at times attempted to argue that the Old Testament supports a multiple person God by using some (very loose) interpretations of Old Testament texts. However, Jews do not interpret those texts in that manner, obviously, and they insist that the Oral Law was also revealed on Mount Sanai and is preserved in the Mishna and Gemara, i.e. the Talmud. I take it that Nabeel Qureshi doesn’t believe that the Oral law preserved in the Talmud was revealed by God on Mount Sanai, but that’s beside the point. The point is that that is the belief of Jews, Jews who live today and who are the descendants of the Israelites.
Therefore, what Nabeel Qureshi should have said is that the Trinity is an elaboration of Old Testament theology as interpreted by Trinitarian Christians, not Jewish theology, the people who determine Jewish theology are Jews.
So Muslims reject the Trinity, and Jews do as well, so either the Jewish and Muslim God is not the same God as the God of the Christians on the basis of the trinity, or the Jewish and Muslim Good is the same God as the God of the Christians despite the Trinity.
In the debates both Volf and Cummings point out that the Muslim rejection of the Trinity is in fact a rejection of a false conception of the Trinity, in both debates Qureshi retorts that Mohammad was trying to reject the Trinity but got the theory wrong. Now let’s think about this for a second. Let’s say there was a proto-Orthodox Christian in the second or third century, he believed Jesus was Yahweh and one with the Father, he had never heard the word Trinity, or the theory of the trinity. Would we say that this Proto-Orthodox Christians was non-Trinitarian? Or Anti-Trinitarian? Probably not, his beliefs are certainly compatible with the trinity. Now let’s say some confused guy comes up to him and says “you should believe the Trinity” and our Proto-Orthodox says “what’s the trinity?” and our confused guy replies “the Trinity is the doctrine that there are three Gods” and the Proto-Orthodox Christian says “Oh, I absolutely do not believe that, I reject the Trinity.” At this point, has the Proto-Orthodox Christian rejected the trinity? No, he wasn’t presented with the trinity, but was he attempting to reject the trinity? Well, he was successfully rejecting the wrong conceptions of the Trinity that was presented to him, as even an Orthodox Trinitarian should.
One cannot attempt to reject something one does not know of. If I reject the doctrine of Universalism because I believe that Universalism means we worship the Universe, I haven’t rejected universalism, I haven’t attempted to reject it, I rejected a doctrine which was wrongly labelled Universalism. The Qur’an doesn’t reject the Trinity; it rejects a doctrine that claims that God is one of three (which is obviously not the trinity) which it mistakenly calls the trinity.