In his Ethics, demonstrated geometrically from axioms and definitions, Spinoza proves that only one substance can exist (since substances are defined as independent, and only one thing can be truly independent). Everything else—minds, bodies, events—are modes or modifications of this one substance.
This substance is both God and Nature, neither transcendent nor personal but immanent and impersonal. The universe doesn't require an external creator; it is self-causing and self-sustaining. Traditional religious language misunderstands reality by imagining God as separate lawgiver when divine law simply is natural law.
This view earned Spinoza excommunication from the Jewish community and charges of atheism from Christians. Yet his system offers a vision of profound unity: humans are not separate from nature or God but expressions of the one substance. Understanding this brings what Spinoza calls "the intellectual love of God"—joy in grasping our place in the infinite whole.
His influence extends through German Idealism to contemporary naturalistic philosophy.
Source
Ethics - Baruch Spinoza