This article looks at the use of the word christianitas at the time of Innocent III (1198–1216 CE) to study how contemporary word use can contribute to the history of a concept. The papal register of letters shows that it is difficult to trace a consistent use of christianitas as a term for the concept of Christendom by Innocent III. In England, France, and Germany the word mainly designated the Christian religion, a personal virtue, or a restricted clerical unit, whereas the Armenians and others tried to invoke the idea of Christendom to rally support for their own political agenda. The constitutions of the IV Lateran Council, where Innocent III gathered ecclesiastical and secular princes from almost all Christian lands to impose his church reform, do not contain the word. It thus seems questionable if christianitas could be employed as the generally accepted term for designating a concept or an actually existing supranational unit.