Spinoza notoriously argues that there is only (and could only be) one substance (Ethics 1p14), “God or nature” (1p29s) and everything else, including human persons, is just a “modification” of the attributes of God (2p10). But he has a very high bar for substance-hood — you have to be self-caused and self-explicating to be a substance (1d3, 1a1), which we plainly aren’t. (Valteri Viljanen observes, “The distinction between substance and mode in Spinoza corresponds to the distinction between substance and accident in Aristotle (“Spinoza’s Ontology,” Cambridge Companion, 57). He (?) even suggests a possible background in Aquinas’s ST 1.29.2.c.: “[T]hose things subsist which exist in themselves, and not in another” (58).) That doesn’t mean that Spinoza denies we exist — we just have to exist “in” another, namely God, if by independent existence we mean what Aristotle and Aquinas seem to mean, namely, self-explication and self-subsistence.
But, apart from the terminological innovations, isn’t this quite close to Augustine, and indeed many other Christian thinkers who take the doctrine of creation out of nothing seriously? Augustine also argues that, strictly speaking, only God is a proper substance and so truly exists (De Trin. 5.3) — creatures exist only by a donation of being from God.
One way of reading Spinoza is as radicalizing the Platonist impulse to press behind the appearances of things to search for their ultimate ground; Aristotelians, by contrast, have always been more at home among medium-sized dry goods than have Platonists.