The Institutes (De Institutis Coenobiorum)

This is John Cassian's The Institutes, his own early fifth century monastic treatise setting out the rules of communal religious life and his analysis of the eight principal vices that would later be reshaped into the seven deadly sins. As the primary text it is the correct source for what Cassian himself argued, in his own structure and sequence. Its limit is that it is one early monastic author's own rule and teaching, addressed to his own monastic community, not a comprehensive history of how monasticism developed more broadly across the late antique Mediterranean, so a claim about that wider history needs a source addressing it directly.