The relationship between the polite and conventional nature of friendly language and the sincerity of the writer’s feelings is a central topic in linguistic and historical research on friendship in epistolary communication. This relationship can be understood in the context of the emotional values and conventionalised emotional practices that characterise the writer’s emotional community. The language of friendship has a significant role in the history of letter writing in religious communities. However, epistolary and emotional practices among religious groups in the modern era remain a rather unexplored filed of research. In this regard, the nineteenth century is of particular interest, as it saw the consolidation of sincerity as a central notion in European standards of letter writing. Bringing together historical pragmatics and the history of emotions, this paper describes the forms and functions of sincerity in the negotiation of friendships between nineteenth-century Catholic churchmen. The article analyses a corpus of letters in Italian and Spanish from the multilingual correspondence of European Benedictine missionaries in Australia between the 1850s and the 1890s. The results of the analysis show that sincerity and emotional self-disclosure, while dependent on the pragmatic conventions of letter writing, belonged to cross-linguistic cultural scripts typical of religious communities.