Why did Spinoza call the single substance of the universe “God”?

In his rationalist system, Benedict de Spinoza identified the single, infinite substance of the universe as "God" because he concluded that there can be only one substance, which is necessarily existent, self-caused, and infinite.12

Spinoza’s metaphysics posits that because God is an infinite being, he must be identical with the world; otherwise, the totality of "God-and-world" would be greater than God alone. Consequently, he argued that "whatever is, is in God, and nothing can be or be conceived without God". In this pantheistic framework, everything that exists is a modification or "mode" of this single substance.312

By defining God as a substance with infinite attributes, Spinoza established that all physical and mental entities are merely different aspects of this one reality. This identification allowed him to move away from supernaturalism toward a view where God is the eternal, "nature-creating" (natura naturans) foundation of all existence, rather than a deity who intervenes in the world.12