This chapter aims to analyse the interactions between Jews and Christians in sixth-century South Arabia, offering some reflections on the broader Late Antique socio-economic and political map. The first part reconstructs the spread of Christianity in South Arabia and the events leading to the massacre of the Christians of Najrān in 523, presenting a comprehensive analysis of this period through a reading of literary and epigraphic material. It argues that economic reasons were the main motivations behind the negus’s invasion of South Arabia and that faith was exploited as a casus belli. Conversions tended to be the cumulative result of socio-economic networks and migrations, as the exchange of ideas followed that of resources. As such, the depiction of the massacre of Najrān as a ‘religious slaughter’ reflects more the ‘religious’ character of the available literary sources than the actual unfolding of the events. The second part of the chapter focuses on the Red Sea Christianities. It examines the religious allegiances of the Aksūmite negus Kālēb and the Ḥimyarite king Abraha, shedding light on the several stages involved in the Christianisation of these two regions and reconstructing the events that led to the collapse of Ḥimyar.