Written over a period of fifteen years, from 400 to 416, St Augustine's book 'On the Trinity' argued against heretics by attempting to show that the Bible contains the doctrine of God as three persons. Medieval theologians valued On the Trinity' because, besides defending the Trinity, Augustine compared the structure of human soul with it and explained how one could get a little understanding of Christian mysteries, or doctrine not completely comprehensible by living humans, by approaching them as riddles posed with symbols. The Benedictine monks of Rochester cathedral would have found a book on how to understand the mysteries very useful to their Bible studies. When this manuscript was copied, the Benedictines had relatively recently replaced the cathedral's secular chaplains, with the arrival of the Norman bishop, Gandulf, in 1077. This flyleaf faces the start of the text. At the bottom is the large formal ownership inscription and anathema of Rochester cathedral; at the top is an informally written note to the effect that the book was written by Humfrey the precentor, and below this a note saying 'pray for him', by a writer who identifies himself as 'Wybarn'.