He was son to a British lord, who to procure him a virtuous education, placed him in his infancy in the monastery of St. Iltutus in Glamorganshire. The surname of Badonicus was given him, because, as we learn from his writings, he was born in the year in which the Britons under Aurelius Ambrosius, or, according to others, under king Arthur, gained the famous victory over the Saxons at Mount Badon, now Bannesdown, near Bath, in Somersetshire. This Bede places in the forty-fourth year after the first coming of the Saxons into Britain, which was in 451. Our saint, therefore, seems to have been born in 494; he was consequently younger than St. Paul, St. Samson, and his other illustrious school-fellows in Wales: but by his prudence and seriousness in his youth he seemed to have attained to the maturity of judgment and gravity of an advanced age. The author of the life of St. Paul of Leon, calls him the brightest genius of the school of St. Iltut. His application to sacred studies was uninterrupted, and if he arrived not at greater perfection in polite literature, this was owing to the want of masters of that branch in the confusion of those times. As to improve himself in the knowledge of God and himself was the end of all his studies, and all his reading was reduced to the study of the science of the saints, the greater progress he made in learning, the more perfect he became in all virtues. Studies which are to many a source of dissipation, made him more and more recollected, because in all books he found and relished only God, whom alone he sought. Hence sprang that love for holy solitude, which, to his death, was the constant ruling inclination of his heart. Some time after his monastic profession, with the consent, and perhaps by the order of his abbot, St. Iltut, he passed over into Ireland, there to receive the lessons of the admirable masters of a religious life, who had been instructed in the most sublime maxims of an interior life, and formed to the practice of perfect virtue, by the great St. Patrick. The author of his Acts compares this excursion, which he made in the spring of his life, to that of the bees in the season of flowers, to gather the juices which they convert into honey. In like manner St. Gildas learned, from the instructions and examples of the most eminent servants of God, to copy in his own life whatever seemed most perfect. So severe were his continual fasts, that the motto of St. John Baptist might in some degree be applied to him, that he scarce seemed to eat or drink at all. A rough hair-cloth, concealed under a coarse cloak, was his garment, and the bare floor his bed, with a stone for his bolster.
By the constant mortification of his natural appetites, and crucifixion of his flesh, his life was a prolongation of his martyrdom, or a perpetual sacrifice which he made of himself to God in union with that which he daily offered to him on his altars. If it be true that he preached in Ireland in the reign of king Ammeric, he must have made a visit to that island from Armorica, that prince only beginning to reign in 560: this cannot be ascribed to St. Gildas the Albanian, who died before that time. It was about the year 527, in the thirty-fourth of his age, that St. Gildas sailed to Armorica, or Brittany, in France: for he wrote his invective ten years after his arrival there, and in the forty-fourth year of his age, as is gathered from his life and writings. Here he chose for the place of his retirement the little isle of Houac, or Houat, between the coast of Rhuis and the island of Bellisle, four leagues from the latter. Houat exceeds not a league in length; the isle of Hoedre is still smaller, not far distant: both are so barren as to yield nothing but a small quantity of corn. Such a solitude, which appeared hideous to others, offered the greatest charms to the saint, who desired to fly, as much as this mortal state would permit, whatever could interrupt his commerce with God. Here he often wanted the common necessaries and conveniences of life; but the greater the privation of earthly comforts was in which he lived, the more abundant were those of the Holy Ghost which he enjoyed, in proportion as the purity of his affections and his love of heavenly things were more perfect. The saint promised himself that he should live here always unknown to men: but it was in vain for him to endeavor to hide the light of divine grace under a bushel, which shone forth to the world, notwithstanding all the precautions which his humility took to conceal it. Certain fishermen who discovered him were harmed with his heavenly deportment and conversation, and made known on the continent the treasure they had found. The inhabitants flocked from the coast to hear the lessons of divine wisdom which the holy anchoret gave with a heavenly unction which penetrated their hearts. To satisfy their importunities, St. Gildas at length consented to live among them on the continent, and built a monastery at Rhuis, in a peninsula of that name, which Guerech, the first lord of the Britons about Vannes, is said to have bestowed upon him. This monastery was soon filled with excellent disciples and holy monks. St. Gildas settled them in good order; then, sighing after closer solitude, he withdrew, and passing beyond the gulf of Vannes, and the promontory of Quiberon, chose for his habitation a grot in a rock, upon the bank of the river Blavet, where he found a cavern formed by nature extended from the east to the west, which on that account he converted into a chapel. However, he often visited this abbey of Rhuis, and by his counsels directed many in the paths of true virtue. Among these was St. Trifina, daughter of Guerech, first British count of Vannes. She was married to count Conomor, lieutenant of king Childebert, a brutish and impious man, who afterwards murdered her, and the young son which he had by her, who at his baptism received the name of Gildas, and was godson to our saint: but he is usually known by the surname of Treuchmour, or Tremeur, in Latin 'Trichmorus. SS. Trifina and Treuchmeur are invoked in the English Litany of the seventh century, in Mabillon. The great collegiate church of Carhaix bears the name of St. Treuchmour: the church of Quim per keeps his feast on the 8th of November, on which day he is commemorated in several churches in Brittany, and at St. Magloire's at Paris. A church situated between Corlai and the abbey of Coetmaloon in Brittany, is dedicated to God under the invocation of St. Trifina.
St. Gildas wrote eight canons of discipline, and a severe invective against the crimes of the Britons, called De Excidio Britanniae, that he might confound those whom he was not able to convert, and whom God in punishment delivered first to the plunders of the Picts and Scots, and afterwards to the perfidious Saxons, the fiercest of all nations. He reproaches their kings, Constantine, (king of the Danmonians, in Devonshire and Cornwall,) Vortipor, (of the Dimetians, in South Wales,) Conon, Cuneglas, and Maglocune, princes in other parts of Britain, with horrible crimes: but Constantine was soon after sincerely converted, as Gale informs us from an ancient Welsh chronicle. According to John Fordun he resigned his crown, became a monk, preached the faith to the Scots and Picts, and died a martyr in Kintyre: but the apostle of the Scots seems to have been a little more ancient than the former. Our saint also wrote an invective against the British clergy, whom he accuses of sloth of seldom sacrificing at the altar &c. In his retirement he ceased not with tears to recommend to God his own cause, or that of his honor and glory, and the souls of blind sinners, and died in his beloved solitude in the island of Horac, (in Latin Horata,) according to Usher, in 570, but according to Ralph of Disse, in 581. St. Gildas is patron of the city of Vannes. The abbey which bears his name in the peninsula of Rhuis, between three and four leagues from Vannes, is of the reformed congregation of St. Maur since the year 1649. The relics of St. Gildas were carried thence for fear of the Normans into Berry, about the year 919, and an abbey was erected there on the banks of the river Indre, which was secularized and united to the collegiate church of Chateauroux in 1623. St. Gildas is commemorated in the Roman Martyrology on the 29th of January. A second commemoration of him is made in some places on the 11th of May, on account of the translation of his relics. His life, compiled from the ancient archives of Rhuis by a monk of that house, in the eleventh century, is the best account we have of him, though the author confounds him sometimes with St. Gildas the Albanian. It is published in the library of Fleury, in Bollandus, p. 954, and most correctly in Mabillon, Act. SS. Ord. Saint Bened. t. 1, p. 138. See also Dom Lobineau, Vies des Saints de Bretagne, (for. an. 1725,) p. 72, and Hist. de la Bretagne, (2 vol. fol. an. 1707) and the most accurate Dom Morice, Memoires sur l'Histoire de Bretagne, 3 vol. fol. in 1745, and Hist. de la Bretagne, 2 vol. fol. an. 1750.