This article seeks to define a theoretical framework for the study of the relation between religion and the political community in the Roman world and to analyze a particular case in point. The first part reviews two prominent theories of religion developed in the last fifty years through the combined efforts of anthropologists and classicists, arguing for their complementary contribution to the understanding of religion's political dimension. It also provides an overview of the approaches of recent scholarship to the relation between religion and the Roman polity, contextualizing the efforts of this article toward a theoretical reframing of the political and institutional elements of ancient Christianity. The second part focuses on the religious legislation of the Theodosian Code, with particular emphasis on the laws against the heretics and their performance in the construction of the political community. With their characteristic language of exclusion, these laws signal the persisting overlap between the borders of the political community and the borders of religion, in a manner that one would expect from pre-Christian civic religions. Nevertheless, the political essence of religion did also adapt to the ecumenical dimension of the empire. Indeed, the religious norms of the Code appear to structure a community whose borders tend to be identical to the borders of the whole inhabited world, within which there is no longer room for alternative affiliations; the only possible identity outside this community is that of the insane, not belonging to any political entity and thus unable to possess any right.