This thesis argues that the origins and evolution of Christian communities in Northern Italy between c. 250 and 381 are comprehensible only within the region's social environment. Whereas previous studies of early Christianity in Italy have sought to explain its origins in terms of modern diocesan structures, this thesis shows that the evidence for this view is untrustworthy and that a new methodology is needed to explain the rise of the church. To this end, the thesis describes the 'north Italian human environment', which consists not just of the physical landscape, but of the social networks within it. This environment allows an understanding of why Christian communities had developed in some places and not in others by c. 300. The development of the church continued to be influenced by this human environment in the fourth century. Christian diffusion remained a partial and variable phenomenon. In the cities Christians found themselves confronted by the adherents of other religions, notably Judaism. Thus, in the fourth century, Christians did not yet dominate the communities in which they lived. Moreover, the active participation in ecclesiastical affairs of emperors after Constantine - particularly the intervention of Constantius II in Italy during the 350s - added a new dimension to the human environment. Such interventions defined how north Italian Christianity came into contact with ecclesiastical and theological affairs throughout the empire. In sum, the history of early Christianity in northern Italy is circumscribed by the social environment within which it developed. This thesis argues that for northern Italy - indeed for the rest of the Mediterranean - a proper understanding of Christian growth can only come from an appreciation of the particular social context of the region within which it occurred.