In 1662, Pieter Balling (d. before 1669), a close friend to Spinoza, published Het Licht op den Kandelaar (translated into English in 1663 as The Light upon the Candlestick), which is often regarded as an exceptionally early “Spinozistic” text. This article, however, analyses the striking parallels with a much older source: the sixteenth-century moral rationalism of Dirck Volckertsz Coornhert (1522–1590). In doing so, I will show that the rationalistic interpretation of spiritualism presented in Het Licht was actually already available long before Balling developed his ideas. I will argue that, besides a deep ideological resemblance, there is sufficient historical evidence to link Balling’s work with Coornhert’s views. Subsequently, I will address the implications of my findings for our understanding of Balling as a radical and, more generally, the rise and spread of radical thought in the seventeenth century, as they help explain why Cartesian and Spinozist rationalism could grow so popular in the Dutch Republic. I will also argue that there is a much broader relevance to these findings, as they highlight the religious overtone of radical rationalism and the Radical Enlightenment as well as their rootedness in radical Reformational currents that developed during the sixteenth century.