St. Gildas the Wise is believed to have been born around the year 517, most likely in northern Britain either in present-day England or Wales. He was the son of Cau (also known as Nau), a man of noble descent, and is thought to have grown up in a large family with several brothers and sisters. Historical writings suggest that one of his brothers, Cuil (or Hueil), was slain by King Arthur, who died in 537 AD. Tradition also holds that Gildas later forgave Arthur for this act.
The life of St. Gildas is known through two separate accounts, though these narratives differ significantly and do not present a single, consistent biography. What is clear, however, is that he lived during a turbulent era when Roman authority had already collapsed in Britain. The Roman legions had been withdrawn earlier by Maximus, who marched them to Rome in his bid to seize imperial power, leaving Britain politically fragmented and vulnerable.
Renowned for his deep piety and strong intellectual formation, Gildas was fearless in condemning the moral failures of kings and leaders. At a time when criticism of rulers often resulted in violent retaliation rather than legal proceedings, he openly rebuked those in power for injustice and corruption.
For many years, Gildas lived as an ascetic hermit on Flatholm Island in the Bristol Channel. There, through intense self-denial and solitude, he embodied the distinctive Celtic ideal of holiness. Welsh tradition also records that during this period he preached to Nemata, the mother of St. David, while she was carrying the future saint.
Around 547, Gildas composed his most famous work, De Excidio Britanniae (The Destruction of Britain). In this text, he offers a brief historical overview of Britain from pre-Roman times and sharply criticizes its rulers for moral decay. He interprets the downfall of British civilization as a direct consequence of the sins of leaders and people alike, clearly intending the work as a moral warning rather than a neutral history.
He later wrote a more extensive work known as the Epistle, a collection of sermons addressing the moral failings of both secular rulers and church leaders. These writings reveal his deep familiarity with Sacred Scripture and classical literature, underscoring his scholarly depth.
St. Gildas was also a powerful and influential preacher. Through his travels to Ireland, he played a key role in missionary activity, contributing to the conversion of many and possibly introducing the anchoritic, or hermit-based, monastic tradition to Irish monks.
Eventually, he withdrew from Llancarfan and settled in Rhuys, in Brittany, where he founded a monastery. Although he wrote several works on monastic life among the earliest such texts in Christian history only one survives today: the Penitential, a guide for abbots on discipline and correction within monastic communities.
St. Gildas died around the year 571 at Rhuys. He is remembered as one of the most significant figures of the early English Church. His influence extended well into the Middle Ages, particularly within the Celtic Christian tradition. Notably, he also holds a unique place in history as the earliest known British author whose writings have survived in substantial form.