In the Dutch debates on Cartesianism of the 1640s, a minority believed that some Cartesian views were in fact Calvinist ones. The paper argues that, among others, a likely precursor of this position is the Aristotelian Franco Burgersdijk (1590-1635), who held a reductionist view of accidents and of the essential extension of matter on Calvinist grounds. It seems unlikely that Descartes was unaware of these views. The claim is that Descartes had two aims in his Replies to Arnauld: to show the compatibility of res extensa and the Catholic transubstantiation but also to differentiate the res extensa from some views of matter explicitly defended by some Calvinists. The association with Calvinism will be eventually used polemically against Cartesianism, for example in France. The paper finally suggests that, notwithstanding the points of conflict, the affinities between the theologically relevant theories of accidents, matter and extension ultimately facilitated the dissemination of Cartesianism among the Calvinists.