Henry More (1614-1687) was probably the most important English philosopher between Hobbes and Locke. Described as the 'hammer' of the Cartesians, More attacked Descartes' conception of spirit as undermining its very intelligibility. This work, which analyses an episode in the evolution of the concept of spiritual substance in early modernity, looks at More's rational theology within the context of the great seventeenth century Cartesian controversies over spirit, soul-body interaction, and divine omnipresence. This work argues that More's new, univocal spirit conception, highly influential upon Newton and Clarke, contributed unwittingly to a slow secularisation process internal to theistic culture. It thus fills a lacuna in scholarship by examining how conceptual changes in early modern metaphysics, as opposed to better researched transformations in moral philosophy, were an additional ingredient in the origins of modern speculative atheism. It also suggests that these controversies are by no means merely of historical interest but represent a resource for contemporary philosophical reflection.