Across the 19th and 20th centuries, the global spread of Protestant Christianity through missionization involved an intense focus on defining the nature of human language in order to discover the languages in which conversion could happen. Using anthropological and linguistic theories, many missionaries came to think of languages of conversion as carriers of tradition, convention, and shared meanings. Ironically, languages that formed during the experience of rapid change and colonization, like pidgins and creoles, were excluded from being considered fully human languages capable of being used in conversion. Using Lutheran and other missionaries in colonial Papua New Guinea as my primary case study, I argue that missionaries consistently ignored potential converts who were swept up in massive economic and political change because the pidginized English spoken by the laborers was not considered capable of sharing Christian truth or sincerity. It was only after the missionaries discovered that the language was being used to deceive that they started to imagine its potential role in conversion. I use this and other examples to show that there are other histories that link language and humanness through invention, deceit, and privacy rather than through convention and shared meanings. These alternate models that emphasize linguistic invention have been used and continue to be used as diagnostic of humanness in a range of contexts, from the colonial past into the technologized present. Connecting these contexts is a Christian concern with attributions of moral agency.