De Excidio et Conquestu Britanniae is a work written in Latin by the probably 6th-century AD British cleric St Gildas. It is a sermon in three parts condemning the acts of Gildas' contemporaries, both secular and religious, whom he blames for the dire state of affairs in sub-Roman Britain. It is one of the most important sources for the history of Britain in the 5th and 6th centuries, as it is the only significant source for the period written by a near contemporary of the people and events described.

Celtic Christianity is a form of Christianity that was common, or held to be common, across the Celtic-speaking world during the Early Middle Ages. Some writers have described a distinct Celtic Church uniting the Celtic peoples and distinguishing them from adherents of the Roman Church, while others classify Celtic Christianity as a set of distinctive practices occurring in those areas. Varying scholars reject the former notion, but note that there were certain traditions and practices present in both the Irish and British churches that were not seen in the wider Christian world.