This chapter explores how casuistry served as a versatile body of knowledge, capable of connecting and adapting the allegedly immutable tradition of Catholic truth to specific social and political changes. Contractual oaths in the context of cross-cultural trade prove to be a case in point to explore the potential of casuistry. In cross-cultural trade, trust was in fact sanctioned by oaths. The problem was, however, that the authority called as a witness to the validity of these oaths was no longer the Christian God, but often the ‘false gods’ of other cultural traditions. Both in Catholic and Protestant contexts the problematic nature of such practices was solved through an innovative application of Christian moral law.