Spinoza was a great man and a wonderfully subversive thinker, but religious/philosophical arguments framed in mathematical terminology don’t exactly make for gripping reading. So I’m just going to quote extensively from Philipp Blom’s book here, as it contains a great summary of his thinking:
Choosing a mathematically inspired way of constructing his argument, with numbered definitions, axioms and propositions, Spinoza argued that only a single substance could exist, a substance infinitely modified to create the world in all its variety. In his Ethics, he states that “God, or substance, consisting of infinite attributes, of which each expresses eternal and infinite essentiality, necessarily exists,” coming to the conclusion that “besides God, no substance can be granted or conceived.” So far, so good for the theologian, who could appreciate that a series of strict definitions and logical conclusions had led Spinoza to proving the necessity of God’s existence.
Spinoza followed Descartes and the Scholastic tradition in defining God as necessarily existing, but he contradicted Cartesian dualism by saying that nothing could exist outside or independently of God, so, by implication, there could not be two realms of the world, mind and matter, but only one single substance of divine origin, infinitely moderated into material and mental phenomena. This not only reversed Descartes’ rescue of theology, but also paved the way for the eighteenth-century materialists, who would argue that the mind is a mere function of the body, not an independent entity.
But Spinoza’s logical rigor had even more troubling implications for theology, particularly, and notoriously, in chapter VI of his Tractatus, in which he wrote about miracles. God, Spinoza held, exists by necessity, is substance, and is perfect. His will is the law of the universe, and his perfection means that the world as such is perfect, since no imperfection can come out of perfection without rendering the originator imperfect. It is therefore impossible that God could will anything to be less than perfect without perverting the very nature of his being. If humans do not perceive the world as perfect—if they point to death, putrefacation and suffering as evident imperfections—it is only because these things are not understandable to them as they are to God. Things may seem imperfect from a person’s limited perspective, but they are necessarily perfect in the eyes of God, who created them as expressions of his own perfection.
From this seemingly abstract and pious analysis, Spinoza drew a scandalous conclusion: Stories of divine miracles, he wrote, must be born out of ignorance or even conscious deception but cannot possibly be used to infer the existence of God. God’s perfection means that the laws of nature must themselves be perfect, too, and it would be impossible for God to intervene in the course of natural events, because any alteration of the course of the universe would mean introducing imperfection, which is incompatible with God’s own perfection. The laws of nature—that is, God’s laws—cannot be contravened, even by God himself.
Here the circle of Spinoza’s virtuoso argument closes. He makes out God to be a being so perfect, so moral, and so necessary that this being could not possibly intervene in the course of nature—indeed, can no longer act at all. Miracles are nothing but misunderstood natural occurrences, he argues. The laws of the universe are synonymous with God’s will and intelligence; God himself becomes a metaphor for necessity, for natural laws.
This is a crucial point. Many interpreters have made Spinoza out to be a pantheist, who sees God’s will, love and Providence in every leaf of grass and every dewdrop, but this is a fundamental misunderstanding. As his contemporary followers and enemies immediately understood, Spinoza’s God is nothing but a particular way of referring to the laws governing the physical world, the only world. One might praise the absolute perfection of this God, but it is soon revealed that no entity, no Creator, no loving or angry Father, no God who saves and punishes is designated by that name. There is only impersonal necessity in a material world; there is no one left to pray to. Spinoza had done nothing less than praise God out of existence.