In the middle of the seventeenth century, scholarship on ancient Stoicism generally understood it to be a form of theism. By the middle of the eighteenth century, Stoicism was widely (though not universally) reckoned a variety of atheism, both by its critics and by those more favourably disposed to its claims. This article describes this transition, the catalyst for which was the controversy surrounding Spinoza's philosophy, and which was shaped above all by contemporary transformations in the historiography of philosophy. Particular attention is paid to the roles in this story played by Thomas Gataker, Ralph Cudworth, J. F. Buddeus, Jean Barbeyrac, and J. L. Mosheim, whose contributions collectively helped to shape the way in which Stoicism was presented in two of the leading reference works of the Enlightenment, J. J. Brucker's Critical History of Philosophy and the Encyclopédie of Diderot and d'Alembert.