Quid sit christianum esse? – what is it to be a Christian? Early in the fifth century an unknown follower of Pelagius set out to answer the question in a letter to a young man. The question was not far below the surface amid the many anxieties at the end of the fourth and the beginning of the fifth centuries. This was the age of Ambrose and Chrysostom, of Jerome and Augustine, as well as of Pelagius and Cassian. In the religious history of Europe, especially of Western Europe, the half century from about 380 to about 430 marks a watershed. On the surface lie the great debates: the debates between pagans and Christians as well as those within the Christian group. In one way or another the debates of these decades all revolved around the question: what is it to be a Christian? What gave the question urgency was the rapid and far-reaching process of christianisation of Roman society which was reaching a climax at this time of dramatic change.