This dissertation traces the metaphysical underpinnings of the Protestant Reformation through a close reading of the work of the Protestant reformer Peter Martyr Vermigli (1499-1562). It is premised on the assumption that all theological reasoning is metaphysical insofar as it simultaneously depends on and conveys a vision of how God and the world relate. This opens the possibility to analyse the implied metaphysics of theological work. The study focuses on four areas of Peter Martyr Vermigli’s work: divine and human agency, grace and justification, the Eucharist, and political theology. It analyses Vermigli’s thought by enquiring what structures of being and causality it displays in each of these areas. The key research question is whether Vermigli’s theology implicitly construes ‘being’ as a neutral category, univocally applying to God’s being as well as created being, or whether it conceives of Divine being as transcendent and pre-eminent, with all other being participating in it. Divine and human causation is moreover construed differently in other of these ontological alternatives. The main argument of this dissertation is that the metaphysical framework sustaining Peter Martyr Vermigli’s thought is complex. When examined in terms of its structures of being and causality, Vermigli’s theology simultaneously inhabits two different metaphysical frameworks, one based on ontological participation and the other on the univocity of being. If Vermigli is representative of the Reformation more broadly – an argument which is made based on recent developments in Calvin and Luther scholarship – then this finding is significant for the hermeneutics of the Protestant reformation in two ways. First, it nuances the Reformation’s role in the genesis of modernity, vis-à-vis certain commentators’ suggestion of a causal link between Reformation thought and modernity, while predicating the latter on a univocal ontology. Secondly, the history and development of Protestantism may be better understood by considering possible long-term effects of the metaphysical complexity at the heart of Reformation thought.