St. Gildas was probably born around 517 in the North of England or Wales. His father's name was Cau (or Nau) and, came from noble lineage, and he most likely had several brothers and sisters. There is evidence that one of his brothers, Cuil (or Hueil), was killed by King Arthur (who died in 537 AD), and it also appears that Gildas may have forgiven Arthur for this.
He lived in a time when the glory of Rome had faded from Britain. The permanent legions had been withdrawn by Maximus, who used them to sack Rome and make himself Emperor.
Gildas was noted in particular for his piety and good educated, and was not afraid to publicly rebuke contemporary monarchs at a time when libel was answered by a sword, rather than a Court order.
Gildas lived for many years as a very ascetic hermit on Flatholm Island in the Bristol Channel. There he established his reputation for that peculiar Celtic sort of holiness that consists of extreme self-denial and isolation. At around this time, according to the Welsh, he also preached to Nemata, the mother of St David, while she was pregnant with the Saint.
In about 547 he wrote a book De Excidio Britanniae (The Destruction of Britain). In this he writes a brief tale of the island from pre-Roman times and criticizes the rulers of the island for their lax morals and blames their sins (and those that follow them) for the destruction of civilization in Britain. The book was avowedly written as a moral tale.
He also wrote a longer work, the Epistle, which is a series of sermons on the moral laxity of rulers and of the clergy. In these Gildas shows that he was well read in the Bible and some other classic works.
He was also a very influential preacher.Because of his visits to Ierland and the great missionary work he did there, he was responsible for the conversion of many on the island, and may be the one who introduced anchorite customs to the monks of that land.
From there he retired from Llancarfan to Rhuys, in Brittany, where he founded a monastery. Of his works on the running of a monastery (one of the earliest known in the Christian Church), only the so-called Penitential, a guide for Abbots in setting punishment, survives.
He died around 571, at Rhuys. The monastery that he had founded became the center of his cult.
He is regarded as being one of the most influential figures of the early English Church. The influence of his writing was felt until well into the middle ages, particularly in the Celtic Church. He is also important to us today as the first British writer whose works have survived fairly intact.