The Ecclesiastical History (Historia Ecclesiastica)

Eusebius of Caesarea was a fourth century bishop and the first major church historian, and this Ecclesiastical History is a primary source recording the early church from the apostolic period through Eusebius's own lifetime, including many documents and earlier writings that survive today only because he quoted them. Its enormous value is exactly that preservation function, without it much early Christian history and many lost texts would be unknown entirely. Its limit is that Eusebius wrote as a partisan of the church he served and, after Constantine, close to imperial power, so his selection and framing of events favors an orthodox and increasingly triumphant narrative, and a critical reader treats his editorial choices as part of the evidence rather than as neutral fact.