While studies in World Christianity have frequently referred to Christianity as a ‘world religion’, this article argues that such a category is problematic. Insider movements directly challenge the category, since they are movements of faith in Jesus that fall within another ‘world religion’ altogether – usually Islam or Hinduism. Rather than being an oddity of the mission frontier, insider movements expose ambiguities already present in World Christianity studies concerning the concept of ‘religion’ and how we understand the unity of the World Christian movement. The article first examines distortions that occur when religion is referred to on the one hand as localised practices which can be reoriented and taken up into World Christianity and, on the other hand, as ‘world religion’, where Christianity is sharply discontinuous with other world systems. Second, the article draws from the field of religious studies, where several writers have argued that the scholarly ‘world religion’ category originates from a European Enlightenment project whose modernist assumptions are now questionable. Third, the particular challenge of insider movements is expanded on – their use of non-Christian cultural-religious systems as spaces for Christ worship, and their redrawing of assumed Christian boundaries. Finally, the article sketches out two principles for understanding Christianity's unity in a way that takes into account the religious (1) as a historical series of cultural-religious transmissions and receptions of the Christian message, which emanates from margins like those being crossed by insider movements, and (2) as a religiously syncretic process of change that occurs with Christ as the prime authority.