Spinoza's Ethics derives pantheism with geometric rigor from a small set of definitions and axioms: there can be only one substance, infinite and self-caused, since anything genuinely infinite could not be limited by another substance existing alongside it, and this one substance, known to finite minds through the attributes of thought and extension, is what Spinoza calls, indifferently, God or Nature, not because he is reducing God to mere matter, a common and largely mistaken charge, but because he is expanding Nature to include everything traditionally attributed to God, infinity, necessity, self-sufficiency, causal power, so that the two names simply pick out one and the same reality described from different angles.
Giordano Bruno, writing a generation before Spinoza and drawing heavily on Hermetic and Neoplatonic sources, argued for an infinite universe animated throughout by a single divine principle, rejecting the finite, earth-centered cosmos of traditional cosmology partly on the theological ground that an infinite God should properly produce an infinite effect, a cosmos without edge or center, a position that put him at odds with church authorities on multiple fronts and contributed to his trial and execution in 1600. Both thinkers were charged with atheism in their own lifetimes, a charge each would have rejected, since neither was denying the existence of the divine but rather denying any divine reality standing apart from and beyond the natural universe itself, which is exactly pantheism's distinctive and, to its critics, most troubling claim.