Fagan, also known by other names including Fugatius, was a legendary 2nd-century Welsh bishop and saint, said to have been sent by the pope to answer King Lucius's request for baptism and conversion to Christianity. Together with his companion St Deruvian, he was sometimes reckoned as the apostle of Britain.

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.