Gildas (English pronunciation: gIl'dəs, Breton: Gweltaz; 450/500-570) — also known as Gildas the Wise or Gildas Sapiens — was a 6th-century British monk best known for his scathing religious polemic De Excidio et Conquestu Britanniae, which recounts the history of the Britons before and during the coming of the Saxons. He is one of the best-documented figures of the Christian church in the British Isles during the sub-Roman period, and was renowned for his Biblical knowledge and literary style. In his later life, he emigrated to Brittany where he founded a monastery known as Saint-Gildas-de-Rhuys.
Differing versions of the Life of Saint Gildas exist, but both agree that he was born in what is now Scotland on the banks of the River Clyde, and that he was the son of a royal family. These works were written in the eleventh and twelfth centuries and are regarded by scholars as unhistorical. He is now thought to have his origins farther south. In his own work, he claims to have been born the same year as the Battle of Mount Badon. He was educated at a monastic centre, possibly Cor Tewdws in South Wales under St Illtud, where he chose to forsake his royal heritage and embrace monasticism. He became a renowned teacher, converting many to Christianity and founding numerous churches and monasteries throughout Britain and Ireland. He is thought to have made a pilgrimage to Rome before emigrating to Brittany, where he took on the life of a hermit. However, his life of solitude was short-lived, and pupils soon sought him out and begged him to teach them. He eventually founded a monastery for these students at Saint-Gildas-de-Rhuys in Brittany, where he wrote De Excidio Britanniae, criticising British rulers and exhorting them to put off their sins and embrace true Christian faith. He is thought to have died at Rhuys, and was buried there.
The First Life of Gildas was written by an unnamed monk at the monastery which Gildas founded in Rhuys, Brittany in the 9th century.