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. Inscriptions on its first page state that it belongs in the cloister of Rochester Cathedral and that it was written in the same cloister by Humphrey, the precentor. In this manuscript, Augustine's prologue is titled in red, 'The Thoughts of St Augustine.' The large letter L (of "Liber", "Book") has a sombre decoration, appropriate to a book for study. Augustine begins his book by stating that it is written in fifteen books over as many years.