Though, at first, being Christian was not a matter of ethnic identity, the Christianisation of the Later Roman Empire, from Constantine to the Edict of Thessalonica (380), gradually connects submission to Roman authority and Christian religion. In this context, one can wonder if the barbarians’ conversion to Christianity had either an ethnical or, at least, a political dimension. During the fourth century, the Empire was seen as a vehicle of Christianity (or sometimes Nicene orthodoxy), facing a barbarian pagan or Arian world. In a certain way, Christianity becomes the new expression of the Roman expansionism in a time when military expansion was over. Missions toward the Barbaricum embrace the language of the Roman domination. Nonetheless, this desire of ruling Christianity in the Barbaricum meets barbarian initiatives that recognize Roman authority over Christianity, even without any political agenda. Even when there is no episcopal hierarchy, the barbarian priests can refer to Roman authority. Two conversions, which are the best documented, show that in this context, conversion to Christianity is one of the crucial steps to become Roman. Both Fritigern around 376 and Fritigil, queen of the Marcoman in 397, while converting to Christianity, chose the faith of the current emperor. For both, also, conversion takes place in a general process of integration to the Roman world. During this process, the choice of Arian or Nicene faith does not match a ‘strategy of distinction’, but rather a strategy of conformity to the Roman authority. During the fourth century, being Arian is not yet a distinctive feature against Romanity. At the end of the century, Christianity turns into a new kind of polis religion, so that becoming Christian is now part of becoming Roman.