This article argues that a relational and trust-based understanding of fides can explain its use and impact in a variety of secular and religious settings, but particularly between members of different religious communities and especially in commercial contracts. The Latin word fides can be translated in a variety of ways, from ‘faith’, ‘trust’ and ‘trustworthiness’, to ‘proof’ and ‘belief’. Within these meanings there are complex religious and legal implications. Most understandings focus on the ways in which the term defines and creates relationships within a community. Contracts from thirteenth- and fourteenth-century Barcelona and Mallorca demonstrate the meaning and significance of the use of the term between merchants and investors from different religious communities. The article provides a new understanding of the place of faith language within the creation of trusting relationships.