American Protestants serving overseas must learn their host communities' "heart" language from the twin moral imperative to minimize their burden on host partners and to embody their faith through their language practice. Language learning thus has moral consequences; not merely proficiency attainment, but faithfulness and calling are at stake. The author, a Protestant himself, conducted an ethnography of Americans serving in a Protestant organization at field sites in Bosnia and Slovenia. Two ideologies emerged from this study and were analyzed as a polycentric system exerting moral authority. These were (a) a "dogmatic" ideology of using the "right" language, the "national" or "heart language" corresponding to a people group, and (b) a "pragmatic" ideology of using "whatever" linguistic resources available to convey their gospel. These ideologies overlapped and shifted over time, with consequences for fieldworkers' longevity, the linguistic ecology of host communities, and Christian understandings of language learning.