Due to its enormous importance, Grotius’s contract doctrine has been extensively investigated by legal historians. This paper seeks to enhance scholarly understanding of this topic by looking at commentaries on De jure belli ac pacis written by German theologians and jurists in the second half of the seventeenth century. The paper focuses on comments concerning promises: the criteria for promises that are binding under natural law; the foundations of the obligation to keep promises; error and duress; and immoral promises. Grotius’s contract doctrine rested on a mélange of sources, both theological and juridical, which were not always consistent. Commentators sought to harmonise them by looking at them from different perspectives: some comments were mainly based on interpretations of ius commune texts, whereas others reflected a moral theological approach. The results drew distinctions (directly or indirectly) between law and moral theology that contributed to the fragmentation of Grotius’s synthesis.