"As Arthur said this, the saintly Dubricius, Archibishop of the city of the Legions, climbed to the top of a hill and cried out in a loud voice: 'You who have been marked with the cross of the Christian faith, be mindful of the loyalty you owe to your fatherland and to your fellow-countrymen! If they are slaughtered as a result of this treacherous behaviour by the pagans, they will be an everlasting reproach to you, unless in the meanwhile you do your utmost to defend them! Fight for your fatherland, and if you are killed suffer death willingly for your country's sake. That in itself is victory and a cleansing of the soul. Whoever suffers death for the sake of his brothers offers himself as a living sacrifice to God and follows with firm footsteps behind Christ Himself, who did not disdain to lay down His life for His brothers. It follows that if any one of you shall suffer death in this war, that death shall be to him as a penance and an absolution for all his sins, given always that he goes to meet it unflinchingly." - Geoffrey of Monmouth, The History of the Kings of Britain ix.4 (Penguin 1966, p. 216)
The legend of king Arthur is just that, of course, but the words attributed to the "saintly Dubricius" presumably reflect those of Geoffrey of Monmouth in the mid-12th-century.