Especially after the 1610s, Tridentine Catholicism forcefully reasserted itself as a prominent political and intellectual force in the Spanish Netherlands. Integrating this reality into accounts of Spanish-Netherlandish science in the 17th century has been a considerable challenge for historians of science. The latter either turned their gazes elsewhere or assumed a fundamental incompatibility between "science" and "religion," thus securing one dominant explanation for the classic thesis that the Spanish Netherlands largely "lost the plot" of the so-called Scientific Revolution after the 1620s. This paper turns to a local debate on Thomas Fienus's embryological theses (1620), which has never been studied, to test the underlying assumption that "science" and "religion" can be taken as two distinct and/or opposed categories of historical analysis. I show that this assumption not only fails to capture historical actors' experiences and understandings, but also that it fails to consider how tensions between medicine and theology were positively productive. First, I argue that medical philosophizing was positively motivated by socio-religious concerns of its own. Second, I show that, far from being a protracted battle between two stable positions, the debate constituted an instance of boundary work, where medical philosophers like Fienus progressively tested and repositioned the theological credentials of their preferred theses. This ushered in the adoption of a probabilistic epistemology that increasingly secured Catholic theology's normative credibility and the pursuit of autonomous natural-philosophical inquiry.