The cultural assimilation of immigrants into the host society is often equated with potential economic success, with religion sometimes seen as a barrier. We investigate the role of ethnic enclaves and churches for the assimilation of Danish Americans. We exploit that this otherwise small and homogeneous group divided itself in the early 1880s into rival Lutheran revivalist camps: so-called “Happy” and “Holy” Danes. The former sought the preservation of Danish culture and tradition, while the latter encouraged assimilation. We use data from the US census, and Danish American church and newspaper archives, and find little difference in communities prior to the 1880s. Subsequently, Danish Americans living in a county with a “Happy” church chose more Danish names for their children. Newspapers read by “Holy Danes” saw a more rapid Anglicization of the language used. Beliefs thus mattered for assimilation, but not for occupational scores, which were identical for both communities.