The essay investigates and delineates the figure of Calvinist theologian Johannes Hoornbeeck (1617-1666), a prominent character of the Nadere Reformatie, or the Dutch “Further Reform”. As Dutch Pietism’s champion, practical theologian, and not least polemical author, Hoornbeeck contributed to Calvinist reflection on the crucial themes of ius ad bellum and Just War. For that matter, Hoornbeeck represents a multifaceted profile, as the first scholar to have used distinctly Rabbinic and Hebraic sources to mould his thought. The Talmudic categories of «voluntary warfare» and «war commanded by God» are in fact used by Hoornbeeck to rethink and blur the typical assumptions of the legitimacy of warfare in Early Modern period.