This chapter explores some ways in which Christian belief and some key aspects of Christian identity were articulated and formed from around AD 300 to 451. It displays the character of the interactions between Christian groups and the imperial authorities by considering two disputes which demonstrate the internal problems caused by emergence from the threat of persecution. These were the Melitian and Donatist disputes. In the last century a number of attempts have been made to treat the dispute as a conflict between the provincial 'nationalism' or 'regionalism' of the Donatists, who were striving to preserve an indigenous form of Christianity, and the imperial authorities desire to impose a universal form of Christianity under closer imperial control. In these disputes, internal Christian debate was prompted by changing relations with the non-Christian world. The most important doctrinal controversy of the fourth century concerned Christians' understanding of God, of the nature of Christ and of the very character of salvation.