Caveat: I am much more familiar with Spinoza’s argument in the Ethics than I am with Islamic theology (which I am only now beginning to learn).
I do not think that it is accurate to say that Spinoza is a “pantheist”––he does not argue that everything is God. Rather, he argues that there is no substance other than God, defining substance as that which has no other cause than itself.
Spinoza begins from Definitions and Axioms, in the manner of a geometer (his model is Euclid).
He begins Book 1 of the Ethics (Of God) with the following Definitions:
“By cause of itself, I understand that of which the essence implies existence, otherwise said, that which the mind is not able to conceive except as existing” [this is my free translation of the Latin: “Per causam sui intelligo id, cujus essentia involvit existentiam, sive id, cujus natura non postest concipi nisi existens”].
The third definition states that “By a substance, I understand that which is in itself and which is conceived through itself [by the mind], which is to say that of which the concept does not require, in order to be conceived, the concept of any other thing.” [again, my free translation].
In D6, he defines ‘God’ as “a being absolutely infinite, i.e. a substance consisting of an infinity of attributes, of which each one expresses an eternal and infinite essence.”
An Attribute is “What the intellect perceives of a substance, as constituting its essence,” (E1D4).
Based on this, I think the argument could be presented as followed:
Our minds perceive God as that which exists through itself and in itself, which cannot be conceived except as existing (necessary existence), and which has no cause other than itself.
According to Spinoza, our minds perceive God as being absolutely infinite, and in each of these infinite attributes, the mind perceives God to be eternal and infinite.
This is another way of saying that there is no being that would be prior to God, that God is absolutely unique and indivisible, that God is uncaused, exists necessarily, and that all things exist and are sustained by through God’s creative power.
It’s important to note that in 17th century Europe, there were two major doctrines regarding the meaning of the term “substance”: Aristotelianism and Cartesianism.
For Aristotle, basically everything that exists that can undergo changes (accidents) without being destroyed is considered to be a “substance”. A substance is something that has both form (essence) and matter; it is a kind of thing, for instance, a man. But for Aristotle, there is no such thing as man ‘in general’ unless there are also actual, particular men, which is to say, men who exist materially. Universals (men, dogs, anything that we can name, really) have no existence except insofar as they are actual. This was a major question for all of medieval philosophy, and was part of Aristotle’s disagreement with Plato.
Descartes breaks quite sharply from this, for reasons which aren’t particularly relevant here. In the Meditations, Descartes distinguishes between two substances: extended things, and thinking things. All material things are modifications of extended substance, but there is nothing in body alone which can account for thought… But the mind, according to Descartes, is not capable of causing the material world to exist as a whole. Moreover, because it is imperfect (i.e. I am not omniscient nor omnipotent)––which I know because I am capable of being in error––he reasons that my mind is not sufficient to explain its own existence. The argument itself gets pretty complicated, but he essentially arrives at the idea that if every idea that the mind conceives of something as existing is an effect of some cause…We have in us the idea of an infinitely perfect being. This has to have come from somewhere (this is all the stuff about objective and formal reality). There must be at least as much reality (read: causal power) in the effect as there is in the cause itself. If I have the idea of an infinitely perfect being, the cause of that idea must be at least as real as our idea of it…
BACK TO SPINOZA:
Spinoza’s argument, often mistaken for ‘pantheism’, is a rectification of Descartes view. Against Descartes’ dualism, he instead argues that the is only one substance (only one necessary existent that is cause of itself), while body and mind are modes of this one substance.
I am somewhat of the persuasion that Spinoza pursues the logic of monotheism to a point that could be mistaken for atheism when he states “God, or Nature”––but Nature is not for Spinoza finite or bounded by time or space. It is absolutely infinite and uncaused, and everything that exists obeys its lawfulness.
Nothing about this says “pantheism” to me, because the only thing that exists necessarily is the One, and there is nothing which even approximates it in its causal power, even if that causality is immanent to what does exist. God and reality itself are one in the same, nothing exists apart from God, and God is not some absent first cause that simply plays itself out mechanically (the way many people think of nature), but rather everything is only because of God.
As a philosopher by training, I think there is something to be said for the rationalist monotheism that can be discerned (with careful reading) in philosophers like Spinoza and Plato that can bring someone who is honest and careful to the threshold of belief (despite longstanding doubts and a cautious disposition).
Spinoza gets himself into a lot of trouble. But what gets him particularly into trouble is the Theological Political Treatise, where he proceeds with a critical-historical reading of the Hebrew Scriptures. It’s partly what got him expelled from the Jewish community in Amsterdam. Spinoza, like most Europeans of his day, was almost completely ignorant about the way that Islam was practiced. But he read the Qu’ran (there was a copy found amongst his effects) and it seems that he held it in similar regard to the New Testament.
All of this being said, having read Spinoza for many years, as I read Spinoza and begin to study the Qu’ran and learn more about Islam as a religion, I find them to be extremely complementary to one another, and in some ways, I credit Spinoza with preparing my mind intellectually, if not spiritually, for recognizing Mohammed as clarifying everything in the Jewish and Christian religions that is most precious and permanent, and realizing that which is most essential and universal in the Abrahamic tradition. Like all philosophical texts, they are not really suitable for everyone. But, as a mostly secular person and a philosopher, I think there’s a deep affinity, but I could be wrong.
I say this as someone who is mostly ignorant of Islamic theology, in a spirit of reverence and respect for Muslims and their beliefs, who shares the commitment to peace, loving-kindness, and concern for living a righteous life.