Gildas' surviving written work, De Excidio Britonum or The Ruin of Britain, is a sermon in three parts condemning the acts of his contemporaries, both secular and religious. The first part consists of Gildas' explanation for his work and a brief narrative of Roman Britain from its conquest under the principate to Gildas' time, a chronicle that relates
The third part begins with the words, "Britain has priests, but they are fools; numerous ministers, but they are shameless; clerics, but they are wily plunderers." Gildas continues his jeremiad against the clergy of his age, but does not explicitly mention any names in this section, and so does not cast any light on the history of the Christian church in this period.
The vision presented in this work of a land devastated by plundering raiders and the misrule of corrupt and venial officials has been readily accepted by scholars for centuries, because not only did it fit the accepted belief of invading, destructive barabrians who destroyed Roman civilization within the bounds of the former empire, but it also explained away the awkward question of why Britain was one of the few parts of the Roman Empire that did not acquire a Romance language, as had France, Spain and Romania. However, the student must remember that Gildas' intent in his writing is to preach to his contemporaries after the manner of an old testament prophet, not to write an account for posterity: while Gildas offers one of the first descriptions of the Hadrian's Wall, he also omits details where they do not contribute to his message. Nonetheless, it remains an important work for not only Medieval but English history for being one of the few works written in Britain to survive from the sixth century.
In De Excidio Britonum, Gildas mentions that the year of his birth was the same year that the Battle of Mons Badonicus took place in. The Annales Cambriae, a chronicle found in one manuscript with a version of the Historia Britonum, gives the year of his death as 570. A biography of Gildas was written by Caradoc of Llancarfan in the 12th century.