Eusebius was a prolific author and controversialist. Large parts of his work no longer survive. Doctrinally, he was not always found to be orthodox, at a time when the details of orthodoxy were still being worked out. His enduring contribution is his Ecclesiastical History, which is long, thorough and scholarly and an indispensable source for the history of the early Church. The Second Readings in the Office of Readings also include, in Advent, an extract from a commentary of his on the book of the prophet Isaiah.