What impact did the rise of Christianity in the fourth century have on the constitution of friendships and social networks in the Roman Empire? How and when did Christian authors use, adapt or avoid traditional concepts of social inclusion in a time of religious change? Did they develop novel strategies of community building? This paper offers a case study to these questions. It aims to analyse Basil of Caesarea's use of the language of friendship as a tool to establish social networks. The main source will be his letters. Basil of Caesarea here contrasts Christian love (agape) with the friendship (philia) rooted in the Graeco-Roman culture. Whereas philia is described as a worldly or bodily relationship, agape is seen as a spiritual bond between adherents of the same Christian faith (cf. Bas. epp. 133 and 154). This paper will argue that Basil purposely distinguishes between two different concepts of social inclusion and makes use of either of them according to the socio- or ecclesio-political context of the letter. Basil of Caesarea thus reacts and adapts to the changing religious circumstances by introducing a new and entirely Christian concept of social inclusion, which he juxtaposes in opposition to the classical notion of friendship.