Next of Kin

The Orthosphere has a post arguing that Islam's god is the closest to the god of Judaism, and that it is is in fact Judaism and not Christianity that is the closest relative to Islam. 

I can't speak to the author's historical claims; you can all speak to his doctrinal ones if you wish. I can say, however, that philosophically he is correct. During the Medieval period there was a time when all three of these Abrahamic religions had adopted a broadly Aristotelian, somewhat Neoplatonic metaphysics following the influence of Avicenna. At that time, in other words, philosophically all three religions believed mostly the same things about God's own nature and our ability to understand or approach it. All three took Aristotle's "Unmoved Mover" to be a reasonable first-pass approximation, and then used Neoplatonic insights to try to tackle the parts of God that prove ineffable, and to the same basic effect: mostly we can only speak negatively about what God isn't ("limited," "bad," etc.), but we can make some positive claims that, whatever God is like, he roots our basic ideas of the good, the worthy, the noble, and so forth. 

The one thing that the philosophers really couldn't come together on, however, was the Trinity. There's actually a pretty good pre-Christian Neoplatonic number theory argument for how One necessarily implies Three; in other words, even before the debate passed into the Abrahamic religions, the subject was already under discussion and fully grounded. Those of you who remember our discussion of the Parmenides might recall some of how this works; it's a genuinely ancient problem about the One and the Many, or whether there is in fact only one thing or many, or what would be necessary for something that is One to give rise to things that are apparently many. 

Nevertheless, by the time we get to the period of Maimonides, who was a contemporary of the Islamic philosopher Ibn Rushd, better known as Averroes -- the latter very regularly cited by Aquinas in the Summa Theologiae as "the Commentator" on Aristotle -- we can see that the Jewish philosopher could find a lot of agreement with several Islamic schools, but found the Christians to be incoherent. Avicenna's arguments, as well as Averroes' commentaries on Aristotle, demonstrated clearly the need for the ultimate mover to be a Unity. 

Having accepted all of that, there just wasn't room for a Trinity. Avicenna gives this argument explicitly; in easy-to-digest form it is this:

Assumption: The ultimate mover could be one thing or more than one.

A: If it were one thing, it would be simple in the sense of having no parts but being just one thing.

B: If it were more than one thing, there would have to be a third kind of thing holding the other things together.

C: In the case of (B), the third kind of thing would actually be prior to the ability of the other things to interoperate.

D: In the case of (C), then, the third kind of thing would be a better candidate for the real ultimate mover than the other things it brings together and unifies.

Sub-Conclusion 1: To reach the real ultimate, we have to go through this cycle to the bottom.

E: A 'third kind of thing' like the one mentioned in (B-D) could either be one thing or several things, but this repeats the cycle.

F: Such a repetition of cycles could happen once or many times, potentially infinitely. 

G: It is not possible to go through an actual infinite series (this is proven twice in Avicenna).

H: Thus, there must be a finite number of cycles before we reach a thing that is simple and serves as the ground for all of the other things.

I: From B-D, we know this ultimately prior thing is the best candidate for the real ultimate mover.

∴: The ultimate mover is one simple thing.

Aquinas accepts the simplicity of God and argues for it right at the beginning of the Summa in Question 3 of the whole work. But he also wants it to be three things. For Maimonides, this whole Christian approach is nonsense: it's basically incoherent. He lists his objections to all the other nearby schools of philosophy, but the Christians are the ones he thinks are just out to sea. 

Islam too insists on the ultimate unity of the One that is its god. Christianity doesn't. In this basic way, the Orthosphere is right: Islam's and Judaism's ideas about God are much closer metaphysically than either are to Christianity's.